Light acquisition sheet and rod, and light receiving device and light emitting device each using the light acquisition sheet or rod

ABSTRACT

A light-trapping sheet of the present invention includes: a light-transmitting sheet  2  having first and second principal surfaces; and a plurality of light-coupling structures  3  arranged in an inner portion of the light-transmitting sheet at a first distance or more and a second distance or more from the first and second principal surfaces, respectively, wherein: each of the plurality of light-coupling structures  3  includes a first light-transmitting layer, a second light-transmitting layer, and a third light-transmitting layer arranged therebetween; a refractive index of the first and second light-transmitting layers is smaller than a refractive index of the light-transmitting sheet  2 ; a refractive index of the third light-transmitting layer is larger than the refractive index of the first and second light-transmitting layers; and the third light-transmitting layer has a diffraction grating parallel to the first and second principal surfaces of the light-transmitting sheet.

TECHNICAL FIELD

The present invention relates to a light-trapping sheet and rod for allowing light-trapping utilizing diffraction, and to a light-receiving device and a light-emitting device using the same.

BACKGROUND ART

Where light is propagated between two light-propagating media of different refractive indices, since there is transmission and reflection of light at the interface, it is typically difficult to transfer, with a high efficiency, light from one light-propagating medium to the other light-propagating medium and maintain this state. A conventional grating coupling method shown in Non-Patent Document No. 1, for example, can be mentioned as a technique for taking light into a transparent sheet from an environmental medium such as the air. FIGS. 32( a) and 32(b) are diagrams illustrating the principle of the grating coupling method, showing a cross-sectional view and a plan view of a light-transmitting layer 20 with a linear grating of a pitch Λ provided on a surface thereof. As shown in FIG. 32( a), if light 23 a of a wavelength λ is allowed to enter the grating at a particular angle of incidence θ, it can be coupled to guided light 23B propagating through the light-transmitting layer 20.

CITATION LIST Non-Patent Literature

-   Non-Patent Document No. 1: Ohmsha Ltd., “Optical Integrated     Circuits”, p 94, p 243, Hiroshi Nishihara, et al.

SUMMARY OF INVENTION Technical Problem

However, according to the method disclosed in Non-Patent Document No. 1, only light that satisfies predetermined conditions can be taken into the light-transmitting layer 20, and light that falls out of the conditions is not taken in. FIG. 32( c) shows a vector diagram of light incident on the grating provided on the light-transmitting layer 20. In FIG. 32( c), circles 21 and are centered about point O, wherein the radius of the circle 21 is equal to the refractive index n₀ of an environmental medium 1 surrounding the light-transmitting layer 20, and the radius of the circle 22 is equal to the equivalent refractive index n_(eff) of the guided light 23B. The equivalent refractive index n_(eff) is dependent on the thickness of the light-transmitting layer 20, and takes a particular value, depending on the waveguide mode, between the refractive index n₀ of the environmental medium 1 and the refractive index n₁ of the light-transmitting layer 20. FIG. 32( d) shows a relationship between the effective thickness t_(eff) and the equivalent refractive index n_(eff) in a case where light propagates in the TE mode through the light-transmitting layer 20. The effective thickness is equal to the thickness of the light-transmitting layer 20 where there is no grating, and if there is a grating, it is the thickness of the light-transmitting layer 20 plus the average height of the grating. Induced guided light has modes such as zeroth, first, second, and so forth, which have different characteristic curves as shown in FIG. 32( d). In FIG. 32( c), point P is a point at which a line drawn from point O along the angle of incidence θ crosses the circle 21, point P′ is the foot of a perpendicular from point P to the x axis, and points Q and Q′ are points at which the circle 22 crosses the x axis. The condition for light coupling in the x-axis positive direction is represented by the length of P′Q being equal to an integral multiple of λ/Λ, and the condition for light coupling in the negative direction is represented by the length P′Q′ being equal to an integral multiple of λ/Λ. Note however that λ is the wavelength of light, and Λ is the pitch of the grating. That is, the condition for light coupling is represented by Expression 1.

$\begin{matrix} {\left\lbrack {{Exp}.\mspace{14mu} 1} \right\rbrack\mspace{661mu}} & \; \\ {{\sin\;\theta} = {{\pm n_{eff}} + {q\frac{\lambda}{\Lambda}}}} & (1) \end{matrix}$

where q is the diffraction order represented by an integer. At an angle of incidence other than θ defined by Expression 1, light is not coupled into the light-transmitting layer 20. Even with the same angle of incidence θ, light is not coupled for different wavelengths.

Note that as shown in FIG. 32( b), for light 23 aa incident on the light-transmitting layer 20 at an azimuthal angle φ that is shifted by an angle φ from the direction of incidence of the light 23 a, the essential pitch of the grating of the light-transmitting layer 20 is Λ/cos φ. Therefore, for the light 23 a incident at a different azimuth, the condition for light coupling can be satisfied even with an angle of incidence θ and a wavelength that are different from those defined by Expression 1. That is, where changes in the azimuth of light incident on the light-transmitting layer 20 are tolerated, the condition for light coupling shown by Expression 1 is somewhat widened. However, incident light cannot be coupled to the guided light 23B over a wide wavelength range for every angle of incidence.

The guided light 23B, while propagating through the grating area, radiates light 23 b′ in the same direction as reflected light of the incident light 23 a. Therefore, even if light is incident at a position far away from an end portion 20 a of the grating and propagates through the light-transmitting layer 20 as the guided light 23B, it attenuates by the time it reaches the end portion 20 a of the grating. Therefore, only the light 23 a that is incident at a position close to the end portion 20 a of the grating can propagate through the light-transmitting layer 20 as the guided light 23B without being attenuated by the radiation. That is, even if the area of the grating is increased in order to couple a large amount of light, it is not possible to allow all the light incident on the grating to propagate as the guided light 23B.

The present invention has been made in order to solve such a problem, and it is an object thereof to provide a light-trapping sheet and rod capable of taking in light of a wide wavelength range from a wide area over a wide angle of incidence. It is also an object to provide a light-receiving device and a light-emitting device using the same.

Solution to Problem

A light-trapping sheet of the present invention includes: a light-transmitting sheet having first and second principal surfaces; and a plurality of light-coupling structures arranged in an inner portion of the light-transmitting sheet at a first distance or more and a second distance or more from the first and second principal surfaces, respectively, wherein: each of the plurality of light-coupling structures includes a first light-transmitting layer, a second light-transmitting layer, and a third light-transmitting layer arranged therebetween; refractive indecies of the first and second light-transmitting layers are smaller than a refractive index of the light-transmitting sheet; a refractive index of the third light-transmitting layer is larger than the refractive indecies of the first and second light-transmitting layers; and the third light-transmitting layer has a diffraction grating parallel to the first and second principal surfaces of the light-transmitting sheet.

A light-trapping rod of the present invention includes: a light-transmitting rod having a principal surface and a circular or elliptical cross section; and a plurality of light-coupling structures arranged in an inner portion of the light-transmitting rod at a first distance or more from the principal surface, wherein: each of the plurality of light-coupling structures includes a first light-transmitting layer, a second light-transmitting layer, and a third light-transmitting layer arranged therebetween; refractive indecies of the first and second light-transmitting layers are smaller than a refractive index of the light-transmitting rod; a refractive index of the third light-transmitting layer is larger than the refractive index of the first and second light-transmitting layers; and the third light-transmitting layer includes a diffraction grating parallel to a central axis of the light-transmitting rod.

A light-receiving device of the present invention includes: a light-trapping sheet set forth above; a protrusion/depression structure or a prism sheet provided on the first principal surface or the second principal surface of the light-trapping sheet; and a photoelectric conversion section for receiving light output from the protrusion/depression structure or the prism sheet.

A light-emitting device of the present invention includes: a light-trapping rod set forth above; and at least one light source provided adjacent to the first principal surface of the light-transmitting rod.

Advantageous Effects of Invention

With a light-trapping sheet and a light-trapping rod of the present invention, light incident on the light-transmitting sheet and the light-transmitting rod enters a light-coupling structure arranged in an inner portion thereof, and is converted by the diffraction grating of the third light-transmitting layer in the light-coupling structure to light that propagates in the direction along the third light-transmitting layer to be radiated from the end face of the light-coupling structure. Since the light-coupling structure is in such a positional relationship that it is parallel to the light-transmitting sheet surface or the rod central axis, and the surface of the light-coupling structure is covered by a low-refractive-index environmental medium such as the air, light that is once radiated is repeatedly totally reflected between the surface of the light-transmitting sheet, the surface of the light-transmitting rod, and surfaces of other light-coupling structures, to be confined within the light-transmitting sheet or the light-transmitting rod. Since the diffraction grating includes various pitches and azimuths, it is possible to take in light over a wide area, over a wide wavelength range, e.g., over the entire visible light, for every angle of incidence.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 (a) is a schematic cross-sectional view showing a first embodiment of a light-trapping sheet according to the present invention, and (b) is a plan view showing the position of a fourth region in the first embodiment.

FIGS. 2 (a) and (b) are a schematic cross-sectional view and a plan view showing a light-coupling structure of the first embodiment, (c) is a cross-sectional view showing light being incident on an end face of the light-coupling structure, (d) is a cross-sectional view showing light being incident on the light-coupling structure with a light-transmitting layer 3 c removed, and (e) is a cross-sectional view showing another configuration example of a light-coupling structure.

FIG. 3 A cross-sectional view showing a structure used in analyzing the light-trapping sheet of the first embodiment.

FIG. 4 Results of an analysis conducted using the structure shown in FIG. 3, wherein (a) to (c) each show the relationship between the angle of incidence of light and the transmittance thereof out of the sheet, and (d) shows the relationship between the groove depth of the diffraction grating and the light take-out efficiency out of the sheet.

FIG. 5 (a) to (e) are diagrams showing light intensity distributions on the sheet cross section under conditions at positions indicated by arrows in FIGS. 4( a) to 4(c).

FIG. 6 Results of an analysis with the structure shown in FIG. 3 where the refractive index of a first light-transmitting layer 3 a and a second light-transmitting layer 3 b is made equal to the refractive index of the light-transmitting sheet, and the refractive index of the third light-transmitting layer 3 c is set to 2.0, wherein (a) to (c) each show the relationship between the angle of incidence and the transmittance thereof out of the sheet, and (d) shows the relationship between the groove depth of the diffraction grating and the light take-out efficiency out of the sheet.

FIG. 7 (a) to (e) are schematic cross-sectional views showing a manufacturing procedure of the light-trapping sheet of the first embodiment.

FIGS. 8 (a) and (b) are schematic plan views each showing a surface pattern of a mold used in manufacturing the light-trapping sheet of the first embodiment.

FIGS. 9 (a) and (b) are a schematic cross-sectional view and a plan view showing a light-coupling structure used in a second embodiment of a light-trapping sheet according to the present invention.

FIG. 10 A cross-sectional view showing a structure used in analyzing the light-trapping sheet of the second embodiment.

FIG. 11 Results of an analysis conducted using the structure shown in FIG. 10, wherein (a) to (c) each show the relationship between the angle of incidence and the transmittance out of the sheet, and (d) shows the relationship between the groove depth of the diffraction grating and the light take-out efficiency out of the sheet.

FIG. 12 Results of an analysis conducted using the structures shown in FIGS. 3 and 10 where the position of the light source is shifted by 5 μm in the x-axis negative direction, wherein (a) to (c) each show the relationship between the angle of incidence of light on the end face of a single light-coupling structure and the transmittance thereof out of the sheet.

FIG. 13 (a) to (e) are schematic cross-sectional views showing a manufacturing procedure of the light-trapping sheet of the second embodiment.

FIGS. 14 (a) and (b) are a schematic cross-sectional view and a plan view showing a light-coupling structure used in a third embodiment of a light-trapping sheet according to the present invention.

FIG. 15 A cross-sectional view showing a structure used in analyzing the light-trapping sheet of the third embodiment.

FIG. 16 Results of an analysis conducted using the structure shown in FIG. 15, wherein (a) to (c) each show the relationship between the angle of incidence and the transmittance out of the sheet, and (d) shows the relationship between the groove depth of the diffraction grating and the light take-out efficiency out of the sheet.

FIG. 17 Results of an analysis conducted using the structures shown in FIGS. 3 and 15 where the position of the light source is shifted by 5 μm in the x-axis negative direction, wherein (a) to (c) each show the relationship between the angle of incidence of light on the end face of a single light-coupling structure and the transmittance thereof out of the sheet.

FIG. 18 (a) to (f) are schematic cross-sectional views showing a manufacturing procedure of the light-trapping sheet of the third embodiment.

FIGS. 19 (a) and (b) are schematic plan views each showing a surface pattern of a mold used in manufacturing the light-trapping sheet of the third embodiment:

FIG. 20 A schematic cross-sectional view showing an embodiment of a light-receiving device according to the present invention.

FIG. 21 A schematic cross-sectional view showing another embodiment of a light-receiving device according to the present invention.

FIG. 22 A schematic cross-sectional view showing another embodiment of a light-receiving device according to the present invention.

FIG. 23 A schematic cross-sectional view showing another embodiment of a light-receiving device according to the present invention.

FIG. 24 A schematic cross-sectional view showing another embodiment of a light-receiving device according to the present invention.

FIG. 25 A schematic cross-sectional view showing an embodiment of a lighting plate according to the present invention.

FIG. 26 A schematic cross-sectional view showing an embodiment of a light-emitting device according to the present invention.

FIGS. 27 (b) and (a) are schematic cross-sectional views parallel to and perpendicular to the central axis showing an embodiment of a light-trapping rod according to the present invention.

FIG. 28 A schematic diagram showing a manufacturing procedure of the light-trapping rod shown in FIG. 27.

FIG. 29 A schematic cross-sectional view showing another embodiment of a light-emitting device of the present invention.

FIG. 30 A cross-sectional view showing light being incident on a cross section of a light-trapping rod of the light-emitting device shown in FIG. 29.

FIG. 31 A schematic cross-sectional view showing invention.

FIGS. 32 (a) and (b) are a cross-sectional view and a plan view of a linear grating for taking in light by a grating coupling method, and (c) and (d) are diagrams showing the principle of the grating coupling method.

DESCRIPTION OF EMBODIMENTS First Embodiment

A first embodiment of a light-trapping sheet according to the present invention will be described. FIG. 1( a) is a schematic cross-sectional view of a light-trapping sheet 51. The light-trapping sheet 51 includes a light-transmitting sheet 2 having a first principal surface 2 p and a second principal surface 2 q, and at least one light-coupling structure 3 provided in the light-transmitting sheet 2.

The light-transmitting sheet 2 is formed by a transparent material that transmits light of a desired wavelength or within a desired wavelength range determined according to the application. For example, it is formed by a material that transmits visible light (wavelength: 0.4 μm or more and 0.7 μm or less). The thickness of the light-transmitting sheet 2 is about 0.03 mm to 1 mm, for example. There is no particular limitation on the size of the first principal surface 2 p and the second principal surface 2 q, and they each have an area determined according to the application.

As shown in FIG. 1( a), the light-coupling structures 3 are arranged in an inner portion of the light-transmitting sheet 2 at a first distance d1 or more and a second distance d2 or more from the first principal surface 2 p and the second principal surface 2 q, respectively. Therefore, in the light-transmitting sheet 2, the light-coupling structure 3 is not provided in a first region 2 a that is in contact with the first principal surface 2 p and has a thickness of the first distance d1, and in a second region 2 b that is in contact with the second principal surface 2 q and has a thickness of the second distance d2, and the light-coupling structure 3 is provided in a third region 2 c arranged between the first region 2 a and the second region 2 b.

The light-coupling structures 3 are three-dimensionally arranged in the third region 2 c of the light-transmitting sheet 2. Preferably, the light-coupling structures 3 are two-dimensionally arranged on a surface parallel to the first principal surface 2 p and the second principal surface 2 q, and a plurality of sets of the two-dimensionally-arranged light-coupling structures 3 are layered together in the thickness direction of the light-transmitting sheet 2.

The light-coupling structures 3 are arranged with a predetermined density in the x,y-axis direction (in-plane direction) and the z-axis direction (thickness direction). For example, the density is 10 to 10³ per 1 mm in the x-axis direction, 10 to 10³ per 1 mm in the y-axis direction, and about 10 to 10³ per 1 mm in the z-axis direction. In order to efficiently take in light illuminating the entirety of the first principal surface 2 p and the second principal surface 2 q of the light-transmitting sheet 2, the density with which the light-coupling structures 3 are arranged in the x-axis direction of the light-transmitting sheet 2, that in the y-axis direction and that in the z-axis direction are preferably independent of one another and uniform. Note however that depending on the application or the distribution of light illuminating the first principal surface 2 p and the second principal surface 2 q of the light-transmitting sheet 2, the arrangement of the light-coupling structures 3 in the light-transmitting sheet 2 may not be uniform and may have a predetermined distribution.

FIGS. 2( a) and 2(b) are a cross-sectional view along the thickness direction of the light-coupling structure 3, and a plan view orthogonal thereto. The light-coupling structure 3 includes the first light-transmitting layer 3 a, the second light-transmitting layer 3 b, and the third light-transmitting layer arranged therebetween. The third light-transmitting layer 3 c includes a diffraction grating 3 d having a linear grating of the pitch Λ provided on the reference plane. The linear grating of the diffraction grating 3.d may be formed by protrusions/depressions provided at the interface between the third light-transmitting layer 3 c and the first light-transmitting layer 3 a or the second light-transmitting layer 3 b, or may be provided inside the third light-transmitting layer 3 c as shown in FIG. 2( e). It may be a grating based on refractive index differences, instead of a grating with protrusions/depressions. In the light-coupling structure 3, the diffraction grating 3 d of the third light-transmitting layer 3 c is arranged in the light-transmitting sheet 2 so as to be parallel to the first principal surface 2 p and the second principal surface 2 q of the light-trapping sheet 51. Herein, the diffraction grating being parallel to the first principal surface 2 p and the second principal surface 2 q means that the reference plane on which the grating is provided is parallel to the first principal surface 2 p and the second principal surface 2 q.

The thicknesses of the first light-transmitting layer 3 a, the second light-transmitting layer 3 b and the third light-transmitting layer 3 c are a, b and t, respectively, and the step (depth) of the linear diffraction grating of the third light-transmitting layer 3 c is d. The surface of the third light-transmitting layer 3 c is parallel to the first principal surface 2 p and the second principal surface 2 q of the light-transmitting sheet 2, and surfaces 3 p and 3 q of the first light-transmitting layer 3 a and the second light-transmitting layer 3 b that are located on the opposite side from the third light-transmitting layer 3 c are also parallel to the first principal surface 2 p and the second principal surface 2 q of the light-transmitting sheet 2.

As will be described below, in order to be able to take in light of different wavelengths incident on the light-trapping sheet, it is preferred that the light-trapping sheet 51 includes a plurality of light-coupling structures 3, and at least two of the plurality of light-coupling structures differ from each other in terms of the direction in which the diffraction grating 3 d extends. Alternatively, is preferred that at least two of the plurality of light-coupling structures 3 differ from each other in terms of the pitch Λ of the diffraction grating 3 d. Alternatively, a combination thereof may be used.

The refractive index of the first light-transmitting layer 3 a and the second light-transmitting layer 3 b is smaller than the refractive index of the light-transmitting sheet 2, and the refractive index of the third light-transmitting layer 3 c is larger than the refractive index of the first light-transmitting layer 3 a and the second light-transmitting layer 3 b. Hereinbelow, it is assumed that the first light-transmitting layer 3 a and the second light-transmitting layer 3 b are the air, and the refractive index thereof is 1. It is also assumed that the third light-transmitting layer 3 c is formed by the same medium as the light-transmitting sheet 2, and they have an equal refractive index.

The surfaces 3 p and 3 q of the first light-transmitting layer 3 a and the second light-transmitting layer 3 b of the light-coupling structure 3 are each a rectangular of which two sides are the lengths W and L, for example, and W and L are 3 μm or more and 100 μm or less. That is, the surfaces of the first light-transmitting layer 3 a and the second light-transmitting layer 3 b of the light-coupling structure 3 are each sized so as to circumscribe a circle having a diameter of 3 μm or more and 100 μm or less. The thickness (a+t+d+b) of the light-coupling structure 3 is 3 μm or less. While the surface (plane) of the light-coupling structure 3 has a rectangular shape as shown in FIG. 2( b) in the present embodiment, it may have a different shape, e.g., a polygonal shape, a circular shape, or an elliptical shape.

The light-trapping sheet 51 is used while being surrounded by an environmental medium. For example, the light-trapping sheet 51 is used in the air. In this case, the refractive index of the environmental medium is 1. Hereinbelow, the refractive index of the light-transmitting sheet 2 is assumed to be n_(s). Light 4 from the environmental medium enters the inside of the light-transmitting sheet 2 through the first principal surface 2 p and the second principal surface 2 q of the light-transmitting sheet 2. An AR coat or anti-reflective nanostructures may be formed on the first principal surface 2 p and the second principal surface 2 q in order to increase the transmittance of the incident light 4. The anti-reflective nanostructures include minute protrusion/depression (or diffraction) structures, such as moth-eye structures, whose pitch and height are ⅓ or less the design wavelength. The design wavelength is the wavelength of light used when designing the various elements so that the light-trapping sheet 51 exhibits a predetermined function. Note that with anti-reflective nanostructures, Fresnel reflection is reduced but total reflection is present.

Hereinbelow, of the light present inside the light-trapping sheet 51, light that satisfies sin θ<1/n_(s) will be referred to as the narrow-angle light (light with propagation angle which is lower than critical angle) and light that satisfies sin θ1/n_(s) as the wide-angle light (light with propagation angle which exceeds critical angle), regarding the angle θ (hereinafter referred to as the propagation angle) formed between the propagation azimuth thereof and the normal to the light-transmitting sheet 2 (a line perpendicular to the first principal surface 2 p and the second principal surface 2 q). In FIG. 1( a), where narrow-angle light 5 a is present inside the light-trapping sheet 51, a portion thereof is converted by a light-coupling structure 3 to wide-angle light 5 b, and this light is totally reflected by the first principal surface 2 p to be wide-angle light 5 c that stays inside the sheet. A portion of the remaining narrow-angle light 5 a′ of the narrow-angle light 5 a is converted by another light-coupling structure 3 to wide-angle light 5 b′, and this light is reflected by the second principal surface 2 q to be wide-angle light 5 c′ that stays inside the sheet. Thus, all of the narrow-angle light 5 a is converted to the wide-angle light 5 b or 5 b′ inside the third region 2 c where the light-coupling structures 3 are arranged.

On the other hand, where wide-angle light 6 a is present inside the light-trapping sheet 51, a portion thereof is totally reflected by the surface of a light-coupling structure 3 to be wide-angle light 6 b, and this light is totally reflected by the first principal surface 2 p to be wide-angle light 6 c that stays inside the sheet. A portion of the remaining light of the light 6 a becomes wide-angle light 6 b′ that passes through the third region 2 c where the light-coupling structures 3 are provided, and this light is totally reflected by the second principal surface 2 q to be wide-angle light 6 c″ that stays inside the light-trapping sheet 51. Although not shown in the figure, there is also wide-angle light that stays inside the sheet while being totally reflected between different light-coupling structures 3 and between the first principal surface 2 p and the second principal surface 2 q, i.e., light that propagates through, while staying in, the first region 2 a, the second region 2 b or the third region 2 c. In this case, there may occur a deviation in the distribution of light propagating through the first region 2 a and the second region 2 b. Where the deviation in the distribution of light in the light-trapping sheet 51 is problematic, it is preferred that one or more fourth region 2 h is provided, in the third region 2 c in the light-transmitting sheet 2, where no light-coupling structure 3 is provided, as shown in FIG. 1( a). That is, the light-coupling structures 3 are arranged only in the third region 2 c excluding the fourth region 2 h. In the light-transmitting sheet 2, the fourth region 2 h connects between the first region 2 a and the second region 2 b. The fourth region 2 h extends from the first region 2 a to the second region 2 b, or in the opposite direction, and the azimuth of an arbitrary straight line passing through the fourth region 2 h is along a larger angle than a critical angle that is defined by the refractive index of the light-transmitting sheet and the refractive index of the environmental medium around the light-transmitting sheet. That is, assuming that the refractive index of the environmental medium is 1 and the refractive index of the light-transmitting sheet 2 is n_(e), the angle θ′ of the direction 2 hx in which the arbitrary straight line passing through the fourth region 2 h extends with respect to the normal to the light-transmitting sheet 2 satisfies sin θ′≧1/n_(s). Herein, a straight line passing through the fourth region 2 h refers to the straight line penetrating the surface at which the fourth region 2 h is in contact with the first region 2 a and the surface at which the fourth region 2 h is in contact with the second region 2 b.

FIG. 1( b) is a plan view of the light-trapping sheet 51, showing the arrangement of the fourth regions 2 h. Preferably, a plurality of fourth regions 2 h are provided in the light-transmitting sheet 2 as shown in FIG. 1( b). Since the fourth region 2 h extends from the first region 2 a to the second region 2 b, or in the opposite direction, at an angle larger than the critical angle, only wide-angle light, of the light propagating through the first region 2 a and the second region 2 b of the light-transmitting sheet 2, can pass from the first region 2 a to the second region 2 b, or in the opposite direction, passing through the fourth region 2 h. Therefore, it is possible to prevent the deviation of the light distribution in the light-trapping sheet 51.

As shown in FIG. 2( a), the narrow-angle light 5 a passes through the surface 3 q of the second light-transmitting layer 3 b, and a portion thereof is converted by the function of the diffraction grating 3 d to guided light 5B that propagates inside the third light-transmitting layer 3 c. The remainder primarily becomes the narrow-angle light 5 a′ to pass through the light-coupling structure 3 as transmitted light or diffracted light, or becomes narrow-angle light 5 r to pass through the light-coupling structure 3 as reflected light. Upon entering the second light-transmitting layer 3 b, there is also the wide-angle light 6 b which is reflected by the surface 3 q, but most of the light can be allowed to pass therethrough if anti-reflective nanostructures are formed on the surfaces 3 q and 3 p.

The coupling to the guided light 5B is the same as the principle of the conventional grating coupling method. Before the guided light 5B reaches an end face 3S of the third light-transmitting layer 3 c, a portion thereof is radiated in the same direction as the narrow-angle light 5 r to be narrow-angle light 5 r′, and the remainder is guided to be radiated from the end face 3S of the third light-transmitting layer 3 c to be the wide-angle light 5 c. On the other hand, the wide-angle light 6 a is totally reflected, at the surface 3 q of the second light-transmitting layer 3 b, and it entirely becomes the wide-angle light 6 b. Thus, wide-angle light incident on the surface of the light-coupling structure 3 (the surface 3 p of the first light-transmitting layer 3 a and the surface 3 q of the second light-transmitting layer 3 b) is reflected, as it is, as wide-angle light, while a portion of narrow-angle light is converted to wide-angle light.

Note that if the length of the diffraction grating 3 d of the third light-transmitting layer 3 c is too long, the guided light 5B is entirely radiated before reaching the end face 3S. If it is too short, the efficiency of coupling to the guided light 5B is insufficient. How easily the guided light 5B is radiated is represented by the radiation loss coefficient α, and the intensity of the guided light 5B is multiplied by a factor of exp(−2αL) at a propagation distance of L. Assuming that the value of α is 10 (1/mm), the light intensity will be multiplied by a factor of 0.8 after propagation over 10 μm. The radiation loss coefficient α is related to the depth d of the diffraction grating 3 d, and it monotonously increases in the range of d≦d_(c) while being saturated in the range of d>d_(c). Where the wavelength of light is λ, the equivalent refractive index of the guided light 5B is n_(eff), the refractive index of the light-transmitting layer 3 c is n₁, and the duty of the diffraction grating 3 d (the ratio of the width of the protruding portion with respect to the pitch) is 0.5, d_(c) is give by Expression 2 below.

$\begin{matrix} {\left\lbrack {{Exp}.\mspace{14mu} 2} \right\rbrack\mspace{661mu}} & \; \\ {d_{c} \approx {\frac{\lambda}{2\pi}\sqrt{n_{eff}^{2} - \left( \frac{n_{1} - 1}{2} \right)^{2}}}} & (2) \end{matrix}$

For example, d_(c)=0.107 μm if λ=0.55 μm, n_(eff)=1.25, and n₁=1.5. In the monotonous increase region, the radiation loss coefficient α is in proportion to d squared. Therefore, the length of the diffraction grating 3 d, i.e., the length of the third light-transmitting layer 3 c (the dimensions W and L) is determined by the radiation loss coefficient α, and is dependent on the depth d of the diffraction grating 3 d. Assuming that by adjusting the depth d, the value of α is set in the range of 2 to 100 (1/mm) and the attenuation ratio to 0.5, W and L will be about 3 μm to 170 μm. Therefore, if W and L are 3 μm or more and 100 μm or less, as described above, it is possible to suppress the radiation loss to obtain a high coupling efficiency by adjusting the depth d.

Table 1 shows the visible light wavelength (λ=0.4 to 0.7 μm) of light that is coupled for the pitch Λ and the angle of incidence θ based on Expression 1, where the equivalent refractive index n_(eff) of the guided light 5B is set to 1.25. Each section of a dotted line is the range for coupling. For example, where the pitch is 0.4 μm, light having a wavelength of 0.4 μm is coupled at θ32-14° and light having a wavelength of 0.7 μm is coupled at θ=30°, thereby giving a visible light coupling range from θ=−14° to θ=30°.

TABLE 1 Angle of incidence θ (degrees) −90 −54 −33 −14 0 5 30 49 90 Pitch 0.18 0.4 Λ 0.20 0.4----0.5 (μm) 0.30 0.4---------------------0.7 0.40 0.4--------------------0.7 0.56 0.4------------0.7 1.60 0.4-----0.7 2.80 0.7

The polarity of the angle of incidence θ is relevant to the light coupling direction. Therefore, if one focuses only on the presence/absence of coupling while ignoring the light coupling direction, covering either the range of angles of incidence from 0 to 90° or from −90 to 0° means that coupling is achieved for every angle of incidence. Therefore, it can be seen from Table 1 that in order for light to be coupled for every visible light wavelength and for every angle of incidence, it is preferred to combine together light-coupling structures 3 including diffraction gratings 3 d having pitches Λ from 0.18 μm to 0.56 μm (from 0° to 90°), or from 0.30 μm to 2.80 μm (from −90° to 0°). Taking into consideration changes in the equivalent refractive index and manufacturing errors occurring when forming the waveguide layer and the diffraction grating, the pitch of the diffraction grating 3 d may be generally 0.1 μm or more and 3 μm or less.

For example, as shown in FIG. 2( b), the pitch of the diffraction grating 3 d is Λ for the narrow-angle light 5 a that is incident in the direction perpendicular to the direction in which the diffraction grating 3 d extends, but the effective pitch of the diffraction grating 3 d for light 5 aa that is incident at an azimuthal angle of p is Λ/cos φ. For example, where the azimuthal angle φ of incidence of the light 5 aa is 0 to 87°, the effective pitch is Λ to 19Λ. Therefore, where Λ=0.18 μm is set, it is possible to realize effective pitches Λ from 0.18 to 2.80 μm depending on the azimuth of incident light even with the same diffraction grating 3 d, and where Λ=0.30 μm is set, it is possible to realize pitches Λ from 0.30 to 2.80 μm. Therefore, it can be seen that it is possible to take in light for every visible light wavelength and for every angle of incidence also by placing light-coupling structures 3 of a single pitch in the light-transmitting sheet 2 while turning the light-coupling structures 3 so that the direction in which the diffraction grating extends (the azimuth of the diffraction grating) varies from 0° to 180°, other than by combining together light-coupling structures 3 including diffraction gratings 3 d having different pitches. Moreover, for a plurality of light-coupling structures 3, the pitch of the diffraction grating 3 d and the direction in which the diffraction grating 3 d extends may both be varied.

Next, light at end faces 3 r and 3 s perpendicular to the surfaces 3 p and 3 q of the light-coupling structure 3 (surfaces extending along the normal direction to the light-transmitting layer 3 b) will be discussed. As shown in FIG. 2( c), possible courses of action for the light incident on the end face 3 r of the light-coupling structure 3 are: to be reflected by the end face 3 r; to be diffracted through the end face 3 r; to be refracted passing through the end face 3 r; and to be guided through the third light-transmitting layer 3 c passing through the end face 3 r. For example, the wide-angle light 6 a which is incident on, and passes through, the end faces of the first light-transmitting layer 3 a and the second light-transmitting layer 3 b is refracted to be narrow-angle light 6 a′. A portion of light 6A which is incident on, and passes through, the end face of the third light-transmitting layer 3 c is converted to guided light 6B which propagates inside the third light-transmitting layer 3 c.

For reference, FIG. 2( d) shows the optical path obtained when the third light-transmitting layer 3 c is removed from the light-coupling structure 3 and the space left by the removal is filled with the same air as the first light-transmitting layer 3 a and the second light-transmitting layer 3 b. When the narrow-angle light 5 a is incident on the surface 3 q of the light-coupling structure 3, if the position of incidence is close to the end face 3 s, it is output through the end face 3 s as the wide-angle light 5 a′ as a result of refraction. When the narrow-angle light 5 a is incident on the end face 3 r of the light-coupling structure 3, it is totally reflected by the end face 3 r. When the wide-angle light 6 a is incident on the end face 3 r of the light-coupling structure 3, it is output from the surface 3 p as the narrow-angle light 6 a′ as a result of refraction, irrespective of the position of incidence. When the wide-angle light 6 a is incident on the surface 3 q of the light-coupling structure 3, it is totally reflected by the surface 3 q.

Thus, for light that is incident on the end faces 3 r and 3 s of the light-coupling structure 3, the behavior is complicated, and even if wide-angle light is incident on the end face, it is not always output as wide-angle light. However, if the size of the surface (W, L) is set to be sufficiently (e.g., 4 times or more) larger than the size of the end face (a+t+d+b), the influence at the end face will be sufficiently small, and then the transmission or the reflection of light at the surfaces 3 p and 3 q can be regarded as being the transmission or reflection behavior of light for the entire light-coupling structure 3. Specifically, if the size of the surface 3 p of the first light-transmitting layer 3 a and the surface 3 q of the second light-transmitting layer 3 b is 4 times or more of the thickness of the light-coupling structure 3, it is possible to sufficiently ignore the influence of light at the end faces 3 r and 3 s of the light-coupling structure 3. Therefore, the light-coupling structures 3 exhibit a function of irreversibly converting narrow-angle light to wide-angle light while maintaining wide-angle light as wide-angle light, and if the density of the light-coupling structures 3 is set to a sufficient density, it is possible to convert all the light incident on the light-trapping sheet 51 to wide-angle light (i.e., light confined within the sheet).

FIG. 3 shows a cross-sectional structure of a light-trapping sheet used in an analysis for confirming the light-trapping effect of the light-trapping sheet 51. A light-trapping sheet including one light-coupling structure was used for the analysis. As shown in FIG. 3, a light source S (indicated by a broken line) having a width of 5 μm was set in parallel at a position of 1.7 μm from the second principal surface 2 q of the light-transmitting sheet 2, and the second light-transmitting layer 3 b having a width of 6 μm was arranged in parallel thereabove at a distance of 0.5 μm, with the third light-transmitting layer 3 c and the first light-transmitting layer 3 a of the same width being arranged thereabove. The first principal surface 2 p of the light-transmitting sheet 2 is located at a position of 2.5 μm from the surface of the first light-transmitting layer 3 a. The positions of the first light-transmitting layer 3 a, the second light-transmitting layer 3 b and the third light-transmitting layer 3 c are shifted side to side based on the angle θ so that a plane wave having a polarization at an angle of 45° with respect to the drawing sheet is output from the light source S at an azimuth forming the angle of θ with respect to the normal to the second principal surface 2 q, and the center of the incident light passes through the center of the surface of the second light-transmitting layer 3 b. The thickness a of the first light-transmitting layer 3 a was set to 0.3 μm, the thickness c of the second light-transmitting layer 3 b to 0.3 μm, the thickness t of the third light-transmitting layer 3 c to 0.4 μm, the depth d of the diffraction grating to 0.18 μm, and the pitch Λ of the diffraction grating to 0.36 μm. The refractive index of the light-transmitting sheet 2 and the third light-transmitting layer 3 c was assumed to be 1.5, and the refractive index of the environmental medium, the first light-transmitting layer 3 a and the second light-transmitting layer 3 b to be 1.0.

FIGS. 4( a) to 4(c) are results of an analysis using a light-trapping sheet having the structure shown in FIG. 3, each showing the relationship between the angle of incidence θ of light from the light source S incident on the light-coupling structure 3 and the transmittance of light that is output to the outside of the light-trapping sheet. The structure used in the analysis was as described above. A two-dimensional finite-difference time-domain method (FDTD) was used in the analysis. Therefore, the analysis results are those with a structure in which the cross section shown in FIG. 3 extends infinitely in the direction perpendicular to the drawings sheet. The transmittance was measured while it was stable, and was defined by the ratio of the integrated value of the Poynting vectors passing through the bottom surface (z=0 μm) and the top surface (z≈8 μm) of the analysis area with respect to the integrated value of the Poynting vectors passing through a closed curved surface surrounding the light source. While there are some calculation results exceeding 100%, it is because of slight errors in the measurement of the Poynting vectors of the light source. FIG. 4( a) shows the calculation results for a case where the wavelength λ of the light source is 0.45 μm, FIG. 4( b) for a case where the wavelength λ is 0.55 μm, and FIG. 4( c) for a case where the wavelength λ is 0.65 μm. Each figure uses the depth d of the diffraction grating as a parameter, and is also plotting the results obtained under a condition where there is no light-coupling structure 3 (a configuration only with the light-transmitting sheet 2 and the light source S).

A comparison between the results obtained in a case where the light-coupling structures 3 are present but the depth d of the diffraction grating is d=0 and the results (Nothing) obtained in a case where there is no light-coupling structure shows that the former has a lower transmittance than the latter in a range within the critical angle (41.8°), and they are both substantially zero for angles greater than or equal to that. The reason why the former has a lower transmittance within the critical angle is because light incident on the surface 3 q of the second light-transmitting layer 3 b is refracted and a portion thereof is output from the end face 3 s as wide-angle light, as described above with reference to FIG. 2( d). Note however that for the former, wide-angle light entering through the end face 3 r of the light-coupling structure 3 is refracted through this surface, and is then refracted through the surface 3 p of the first light-transmitting layer 3 a to be narrow-angle light inside the light-transmitting sheet 2, as described above again with reference to FIGS. 2( c) and 2(d). Therefore, for a structure where d=0, there is conversion to wide-angle light while there is also conversion to narrow-angle light, and it can be said that the light-trapping effect as a whole is small.

On the other hand, a comparison between the results for a case where the depth of the grating is d=0.18 μm and the results for a case where d=0 shows that although the transmittance of the former is substantially close to that of the latter, the transmittance drops at positions indicated by arrows a, b, c, d and e. FIG. 4( d) shows the standard value (a value obtained by division by 90) of a value obtained by integrating each of the curves of FIGS. 4( a), 4(b) and 4(c) for the angle of incidence θ, using the depth d of the diffraction grating as a parameter. Since the analysis model is two-dimensional, the integrated value is equal to the efficiency with which light in the light-confining sheet is taken out of the sheet. With any wavelength, the take-out efficiency decreases as d increases (at least for the comparison between d=0 and d=0.18). This represents the light-trapping effect by a single light-coupling structure. This effect can be accumulated, and by increasing the number of light-coupling structures, it is possible to eventually confine all the light. Note that while this analysis is a two-dimensional model, there is always incident light that satisfies Expression 1, which is the coupling condition, for an arbitrary azimuthal angle φ shown in the plan view of FIG. 2(a) in an actual model (three-dimensional model), and therefore the transmittance curves shown in FIG. 4 will drop for the entire range of the angle of incidence θ, rather than for the local range such as the arrows a, b, c, d and e, thus increasing the light-trapping effect of the light-coupling structures.

FIG. 5 shows light intensity distribution diagrams in the light-trapping sheet under conditions indicated by arrows a, b, c, d and e of FIG. 4. Specifically, FIG. 5( a) shows the results where the wavelength is λ=0.45 μm and θ=5°, FIG. 5( b) shows the results where the wavelength is λ=0.55 μm and θ=0°, FIG. 5( c) shows the results where the wavelength is λ=0.55 μm and θ=10°, FIG. 5( d) shows the results where the wavelength is λ=0.65 μm and θ=10°, and FIG. 5( e) shows the results where the wavelength is λ=0.65 μm and θ=20°.

For the conditions and the angles of incidence shown in FIGS. 5( a) and 5(b), since the refractive index of the third light-transmitting layer 3 c is higher than the refractive index of the first light-transmitting layer 3 a and the second light-transmitting layer 3 b surrounding the third light-transmitting layer 3 c, the third light-transmitting layer 3 c functions as a waveguide layer, and the incident light is coupled to the guided light propagating inside the third light-transmitting layer 3 c by the function of the diffraction grating, with the light being radiated into the light-transmitting sheet 2 from the end faces 3 r and 3 s of the third light-transmitting layer 3 c. The radiated light is wide-angle light, and is totally reflected by the first principal surface 2 p and the second principal surface 2 q of the light-transmitting sheet 2 to be confined within the light-transmitting sheet 2. Also for the conditions and the angles of incidence shown in FIGS. 5( c), 5(d) and 5(e), the incident light is coupled to the guided light propagating inside the third light-transmitting layer 3 c by the function of the diffraction grating, with the light being radiated into the sheet from the end face 3 r of the third light-transmitting layer 3 c. The radiated light is wide-angle light, and is totally reflected by the first principal surface 2 p and the second principal surface 2 q of the light-transmitting sheet 2 to be confined within the light-transmitting sheet 2. Note that in FIGS. 5( a), 5(c) and 5(e), the radiated light is divided into two, and the coupled light is guided light of the first-order mode whose phase is reversed above and below the cross section of the waveguide layer. On the other hand, in FIGS. 5( b) and 5(d), the radiated light is in an undivided state, and the coupled light is guided light of the zeroth-order mode.

FIG. 6 shows results of an analysis using the structure shown in FIG. 3 where the refractive index of the first light-transmitting layer 3 a and the second light-transmitting layer 3 b is made to coincide with the refractive index of the light-transmitting sheet 2, and the refractive index of the third light-transmitting layer 3 c is changed to 2.0. The other conditions are the same as those when the analysis results shown in FIG. 4 were obtained. FIG. 6( a) shows the results where the wavelength of the light source is λ=0.45 μm, FIG. 6( b) shows the results where the wavelength is λ=0.55 μm, and FIG. 6( c) shows the results where the wavelength is λ=0.65 μm. A comparison between the results where the depth of the grating is d=0.18 μm and the results where d=0 shows that the transmittance of the former drops at positions of arrows a, b, c, d, e and f, as compared with that of the latter. This is for the same reason as described above with reference to FIG. 4. However, in the region above the critical angle, the latter comes to the vicinity of zero whereas the former is substantially floating. This is because light of an angle of incidence above the critical angle diffracts through the diffraction grating of the light-coupling structure 3, and a portion thereof is converted to narrow-angle light in the sheet. FIG. 6( d) shows the standard value (a value obtained by division by 90) of a value obtained by integrating each of the curves of FIGS. 6( a), 6(b) and 6(c) for the angle of incidence θ, using the groove depth d as a parameter. For some conditions, an increase in d rather increased the take-out efficiency, thereby failing to obtain the light-trapping effect. This indicates that the characteristics in the region above the critical angle cancel out the effects at the positions of the arrows a, b, c, d, e and f.

A comparison between analysis results of FIGS. 4 and 6 shows that the transmittance is successfully made zero above the critical angle in FIG. 4. A comparison between the results where the depth of the grating is d=0.18 μm and the results where d=0 shows there is no difference in the region above the critical angle, and they are both substantially zero. This is because the refractive index of the first light-transmitting layer 3 a and the second light-transmitting layer 3 b is set to be smaller than the refractive index of the light-transmitting sheet 2, resulting in total reflection at the surface 3 q which is the interface between the second light-transmitting layer 3 b and the light-transmitting sheet 2, whereby light of a large angle of incidence cannot enter the diffraction grating in the light-coupling structure 3, and there is no diffracted light caused by the diffraction grating. Thus, it can be seen that it is preferred with the light-coupling structure 3 that in order for the third light-transmitting layer 3 c to be a light guide layer, the refractive index thereof is larger than the refractive index of the first light-transmitting layer 3 a and the second light-transmitting layer 3 b, and in order for wide-angle light not to enter the third light-transmitting layer 3 c, the refractive index of the first light-transmitting layer 3 a and the second light-transmitting layer 3 b is smaller than the refractive index of the light-transmitting sheet 2. It can also be seen that in order to decrease the critical angle for the total reflection between the light-transmitting sheet 2 and the light-coupling structure, the difference between the refractive index of the first light-transmitting layer 3 a and the second light-transmitting layer 3 b and the refractive index of the light-transmitting sheet is preferably large, and the refractive index of the first light-transmitting layer 3 a and the second light-transmitting layer 3 b is preferably 1, for example.

Thus, with the light-trapping sheet of the present embodiment, light incident on the first principal surface and the second principal surface of the light-transmitting sheet at various angles becomes narrow-angle light and enters a light-coupling structure arranged inside the light-transmitting sheet, and a portion thereof is converted by the diffraction grating in the light-coupling structure to guided light that propagates inside the third light-transmitting layer and is radiated from the end face of the light-coupling structure to be wide-angle light. Because the pitch of the diffraction grating varies and the azimuth of the diffraction grating varies from one light-coupling structure to another, this conversion is achieved for every azimuth over a wide wavelength range, e.g., over the entire visible light range. Since the length of the diffraction grating is short, it is possible to reduce the radiation loss of the guided light. Therefore, narrow-angle light present inside the light-transmitting sheet is all converted to wide-angle light by a plurality of light-coupling structures. Since the refractive index of the first and second transmission layers of the light-coupling structure is smaller than the refractive index of the light-transmitting sheet, the wide-angle light is totally reflected by the surface of the light-coupling structure, and the light is repeatedly totally reflected between the surfaces of other light-coupling structures and the surface of the light-transmitting sheet, thus being confined within the light-transmitting sheet. Thus, the light-coupling structure irreversibly converts narrow-angle light to wide-angle light, while maintaining wide-angle light in the out-of-critical-angle state. Therefore, if the density of the light-coupling structures is set to a sufficient density, it is possible to convert all the light incident on the light-trapping sheet to wide-angle light, i.e., light confined within the sheet.

The light-trapping sheet 51 can be manufactured by the following method, for example. FIGS. 7( a) to 7(e) are schematic cross-sectional configuration views showing a manufacturing procedure of the light-trapping sheet 51, and FIGS. 8( a) and 8(b) are schematic plan views each showing a pattern of a mold surface for producing the sheet.

In FIGS. 8( a) and 8(b), rectangular minute structures 25A and 25B of the same size are two-dimensionally arranged, for example, on the surfaces of molds 25 a and 25 b. The arrangement of the minute structures 25A on the mold 25 a and the arrangement of the minute structures 25B on the mold 25 b are equal. In the present embodiment, the minute structures 25A and 25B are protrusions. The height of the minute structures 25A is the dimension b of FIG. 2( a), and the height of the minute structures 25B is equivalent to the dimension a. While the surface of the minute structure 25B is a plane, a linear diffraction grating having a height of d and a pitch of Λ is formed on the surface of the minute structure 25A, and the azimuth of the diffraction grating (the direction in which the depressed portion or the protruding portion extends) varies from one minute structure 25A to another. While gratings of azimuths of 45° intervals, i.e., 0°, 45°, 90° and 135°, are arranged regularly in FIG. 8, it is preferred in practice that gratings are arranged with an equal frequency at azimuths of smaller intervals, e.g., 30° or 15°.

As shown in FIG. 7( a), with a thin layer of a spacer agent applied on the surface of the mold 25 b, a transparent resin sheet 24 is laid on the surface of the mold 25 b, and the mold 25 a is arranged on the sheet, pressing the resin sheet 24 arranged between the mold 25 b and the mold 25 b while the minute structures 25B and the minute structures 25A are aligned with each other.

As shown in FIG. 7( b), the mold 25 a is lifted, thereby peeling the resin sheet 24 off the mold 25 b, and the resin sheet 24 is pressed against a resin sheet 24 a with a thin layer of an adhesive applied on the surface thereof as shown in FIG. 7( c), thereby bonding together the resin sheet 24 and the resin sheet 24 a. As shown in FIG. 7( d), an adhesive is applied in a thin layer on the bottom surface of the resin sheet 24 a, and it is pressed against similarly-formed resin sheets 24′ and 24′a while ignoring the alignment therebetween, thus bonding them together.

As shown in FIG. 7( e), the mold 25 a is lifted while the resin sheet 24′a is secured, thereby peeling the resin sheets 24, 24 a, 24′ and 24′a as a whole off the mold 25 a.

Thereafter, the resin sheets 24, 24 a, 24′ and 24′a are replaced by the resin sheets 24′ and 24′a of FIG. 7( d), and these steps are repeated, thereby producing the third region 2 c of the light-transmitting sheet 2 shown in FIG. 1( a). Resin sheets to be the first region 2 a and the second region 2 b of the light-transmitting sheet 2 are bonded to the front surface and the reverse surface of the third region 2 c of the light-transmitting sheet 2, thereby completing the light-trapping sheet 51 shown in FIG. 1( a). While an adhesive is used for the bonding between resin sheets in the present embodiment, the surfaces of the resin sheets may be heated so as to weld together the resin sheets, instead of using an adhesive. Anti-reflective nanostructures may be formed in advance on the surface of the resin sheet 24 a and the resin sheets to be the first region 2 a and the second region 2 b.

Second Embodiment

A second embodiment of a light-trapping sheet according to the present invention will be described. A light-trapping sheet 52 of the present embodiment is different from the light-coupling structure of the first embodiment in terms of the structure at the end face of the light-coupling structure. Therefore, the description hereinbelow will focus on the light-coupling structure of the present embodiment.

FIGS. 9( a) and 9(b) schematically show a cross-sectional structure and a planar structure of a light-coupling structure 3′ along the thickness direction of the light-trapping sheet 52. As shown in FIGS. 9( a) and 9(b), a depressed portion 3 t having a depth of e is provided on the end faces 3 r and 3 s of the light-coupling structure 3′. The cross section of the depressed portion 3 t has a width that is tapered inwardly. Therefore, in the light-coupling structure 3′, the thickness of the first light-transmitting layer 3 a and that of the second light-transmitting layer 3 b decrease toward the outer edge side away from the center of the light-coupling structure 3′. The surfaces 3 p and 3 q are flat as they are in the first embodiment.

FIG. 10 shows a cross-sectional structure of a light-trapping sheet used in an analysis for confirming the light-trapping effect of the light-trapping sheet 52 including the light-coupling structure 3′. The light-coupling structure and the light source are arranged at just the same positions as the corresponding elements in the structure used in the analysis in the first embodiment (FIG. 3).

FIGS. 11( a) to 11(c) show results of an analysis using a light-trapping sheet having the structure shown in FIG. 10, each showing the relationship between the angle of incidence θ of light from the light source S incident on the light-coupling structure 3′ and the transmittance of light that is output to the outside of the light-trapping sheet. The same method as that of the first embodiment was used for the analysis. FIG. 11( a) shows the results where the wavelength of the light source is λ=0.45 μm, FIG. 11( b) shows the results where the wavelength is λ=0.55 μm, and FIG. 11( c) shows the results where the wavelength is λ=0.65 μm. Each figure uses the depth d of the diffraction grating as a parameter, and is also plotting the results obtained under a condition where there is no light-coupling structure (a configuration only with the light-transmitting sheet 2 and the light source S).

A comparison between the results obtained in a case where the light-coupling structures 3′ are present but the depth of the diffraction grating is d=0 and the results (Nothing) obtained in a case where there is no light-coupling structure shows that the former is smaller than the latter in a range within the critical angle (41.8°), and they are both zero for angles greater than or equal to that. The reason why the former is smaller within the critical angle is because light incident on the surface 3 q of the second light-transmitting layer 3 b is refracted and a portion thereof is output from the right side face (the right side face of the third light-transmitting layer 3 c) as wide-angle light, as described above with reference to FIG. 2( d).

On the other hand, a comparison between the results for a case where the depth of the grating is d=0.18 μm and the results for a case where d=0 shows that although the transmittance of the former is substantially close to that of the latter, the transmittance drops at positions indicated by arrows a, b, c, d and e. These positions correspond to conditions under which light is coupled to the guided light. FIG. 11( d) shows the standard value (a value obtained by division by 90) of a value obtained by integrating each of the curves of FIGS. 11( a), 11(b) and 11(c) for the angle of incidence θ, using the groove depth d as a parameter. Since the analysis model is two-dimensional, the integrated value is equal to the efficiency with which light in the sheet is taken out of the sheet. With any wavelength, the take-out efficiency decreases as d increases (at least for the comparison between d=0 and d=0.18). This represents the light-trapping effect by a single light-coupling structure, as with the analysis results in the first embodiment. This effect can be accumulated, and by increasing the number of light-coupling structures, it is possible to confine all the light. Note that while this analysis is a two-dimensional model, there is always incident light that satisfies Expression 1, which is the coupling condition, for an arbitrary azimuthal angle φ shown in the plan view of FIG. 2( a) in an actual three-dimensional model, and therefore the transmittance curves shown in FIG. 11 will drop for the entire range of the angle of incidence θ, rather than for the local range such as the arrows a, b, c, d and e, thus increasing the light-trapping effect of the light-coupling structures. The drops at positions of arrows b, c, d and e are smaller as compared with those of the analysis results of the first embodiment because the length of the grating (coupling length) is made smaller in the analysis model of this embodiment.

FIG. 12 shows results of an analysis of the second embodiment, each showing the relationship between the angle of incidence θ of light on the end face of a single light-coupling structure and the transmittance thereof out of the light-trapping sheet. In the analysis conditions used, only the position of the light source S is shifted by 5 μm in the x-axis negative direction from the conditions of FIG. 10 or FIG. 3. FIG. 12( a) shows a case where the wavelength of the light source is λ=0.45 μm, FIG. 12( b) a case where the wavelength is λ=0.55 μm, and FIG. 12( c) a case where the wavelength is λ=0.65 μm, wherein each figure shows a comparison between the model of this embodiment and the model of the first embodiment, and is also plotting the results obtained under a condition where there is no light-coupling structure (a configuration only with the light-transmitting sheet 2 and the light source S).

A comparison between the results for the model of the second embodiment and the results (Nothing) obtained in a case where there is no light-coupling structure shows that they substantially coincide with each other in both cases within the critical angle (41.8° or less), but the latter is substantially zero and the former substantially floats from zero outside the critical angle (41.8° or more). The former floats outside the critical angle because, as described above with reference to FIGS. 2( c) and 2(d), light incident on the end face of the first light-transmitting layer 3 a and the second light-transmitting layer 3 b of the light-coupling structure refracts, and then becomes narrow-angle light and is output from the first principal surface 2 p. In contrast, in the analysis results for the model of the second embodiment, the floating outside the critical angle is partially suppressed. This is because the first light-transmitting layer 3 a and the second light-transmitting layer 3 b account for no area on the end face of the second embodiment, and the refraction at the end face is somewhat suppressed. Therefore, the second embodiment is a configuration such that the influence at the end face (the phenomenon that wide-angle light is converted to narrow-angle light) can be ignored more than in the first embodiment, and can be said to be a configuration having a greater light-trapping effect. Note that in FIG. 12, the length of the light source is set to 5 μm. Increasing this length will increase the proportion of a component that that deviates from the end face of the light-coupling structure and is incident directly on the first principal surface 2 p to be totally reflected or is totally reflected at the surface 3 q of the light-coupling structure, thus reducing the floating outside the critical angle. If the length of the light source is set to 20 μm, which is 4 times more, while the light-coupling structure is set to be about 21 μm, only the floating outside the critical angle, of the end face incidence characteristics, is reduced to about ¼.

FIG. 13 is a schematic cross-sectional view showing an example of a production procedure for the light-trapping sheet 52 of the present embodiment. The light-trapping sheet 52 can be manufactured by using a similar procedure to that of the first embodiment, while providing slopes 25A′ and 25B′ at the outer edge portions of the minute structures 25A and 25B of the molds 25 a and 25 b. Except for the shapes of the molds 25 a and 25 b being different, the light-trapping sheet 52 of the present embodiment can be manufactured in a similar manner to the light-trapping sheet 51 of the first embodiment, and therefore the manufacturing procedure will not be described in detail.

Third Embodiment

A third embodiment of a light-trapping sheet according to the present invention will be described. A light-trapping sheet 53 of the present embodiment is different from the light-coupling structure of the second embodiment in terms of the structure at the end face of the light-coupling structure. Therefore, the description hereinbelow will focus on the light-coupling structure of the present embodiment.

FIGS. 14( a) and 14(b) schematically show a cross-sectional structure and a planar structure of a light-coupling structure 3″ along the thickness direction of the light-trapping sheet 53. As shown in FIGS. 14( a) and 14(b), on the surfaces 3 p and 3 q of the light-coupling structure 3″, tapered portions 3 u and 3 v are provided across areas having the width e adjacent to the end faces 3 r and 3 s. Therefore, the thicknesses of the first light-transmitting layer 3 a and the second light-transmitting layer 3 b are decreased toward the outer edge side away from the center of the light-coupling structure 3″ while maintaining the flatness of the interface between the first light-transmitting layer 3 a and the second light-transmitting layer 3 b and the third light-transmitting layer 3 c.

FIG. 15 shows a cross-sectional structure of a light-trapping sheet used in the analysis for confirming the light-trapping effect of the light-trapping sheet 53 including the light-coupling structure 3″. The light-coupling structure and the light source are provided at just the same positions as those in the structure used in the analysis in the first embodiment (FIG. 3).

FIGS. 16( a) to 16(c) show results of an analysis using a light-trapping sheet having the structure shown in FIG. 15, each showing the relationship between the angle of incidence θ of light from the light source S incident on the side of the light-coupling structure 3′ and the transmittance of light that is output to the outside of the light-trapping sheet. The same method as that of the first embodiment was used for the analysis. FIG. 16( a) is for a case where the wavelength of the light source is λ=0.45 μm, FIG. 16( b) for a case where the wavelength is λ=0.55 μm, and FIG. 16( c) for a case where the wavelength is λ=0.65 μm, wherein each figure uses the depth d of the diffraction grating as a parameter, and is also plotting the results obtained under a condition where there is no light-coupling structure (a configuration only with the light-transmitting sheet 2 and the light source S).

A comparison between the results obtained in a case where the light-coupling structures are present but the depth of the grating is d=0 and the results (Nothing) obtained in a case where there is no light-coupling structure shows that the former is smaller than the latter in a range within the critical angle (41.8°), and the latter is zero for angles greater than or equal to the critical angle, whereas floating remains for the former in the range up to 55°. The reason why the former is smaller within the critical angle is because light incident on the surface 3 q of the second light-transmitting layer 3 b is refracted and a portion thereof is output from the right side face (the right side face of the third light-transmitting layer 3 c) as wide-angle light, as described above with reference to FIG. 2( d). There are two possible reasons for the former to float for angles greater than or equal to the critical angle. First, the surface 3 q of the second light-transmitting layer 3 b is sloped toward the outer edge portion, whereby a portion of light exceeding the critical angle can be incident on the surface 3 q of the second light-transmitting layer 3 b within the critical angle, and this light diffracts through the grating inside the light-coupling structure to be narrow-angle light. Second, the thickness of the second light-transmitting layer 3 b is too small in the outer edge portion, and a portion of light exceeding the critical angle passes into the inside of the light-coupling structure in the form of evanescent light, and this light diffracts through the grating to be narrow-angle light.

On the other hand, a comparison between the results for a case where the depth of the diffraction grating is d=0.18 μm and the results for a case where d=0 shows that although the transmittance of the former is substantially close to that of the latter, the transmittance drops at positions of arrows a, b, c, d and e. These positions correspond to conditions under which light is coupled to the guided light, and the light is guided, after which it is radiated from the end face of the third light-transmitting layer 3 c to be wide-angle light. This radiated light falls within the range of about ±35° about a propagation angle of 90° (x-axis direction) (see FIG. 5).

In FIG. 16, the floating of transmitted light is suppressed at the angle of incidence of 55° or more, and it becomes substantially zero, indicating that light to be guided light and radiated becomes wide-angle light (light whose propagation angle is 55° or more) that is repeatedly totally reflected and stays inside the sheet. Note that as the surface 3 p of the first light-transmitting layer 3 a and the surface 3 q of the second light-transmitting layer 3 b are sloped toward the outer edge portion, the propagation angle of light that is totally reflected at these surfaces increases and decreases depending on the slope direction, but since they occur with the same probability, it is possible to maintain substantially the same propagation angle as a whole.

FIG. 16( d) shows the standard value (a value obtained by division by 90) of a value obtained by integrating each of the curves of FIGS. 16( a), 16(b) and 16(c) for the angle of incidence θ, using the groove depth d as a parameter. Since the analysis model is two-dimensional, the integrated value is equal to the efficiency with which light in the sheet is taken out of the sheet. With any wavelength, the take-out efficiency decreases as d increases (at least for the comparison between d=0 and d=0.18). This represents the light-trapping effect by a single light-coupling structure, as with the analysis results of the first embodiment. This effect can be accumulated, and by increasing the number of light-coupling structures, it is possible to confine all the light. Note that while this analysis is a two-dimensional model, there is always incident light that satisfies Expression 1, which is the coupling condition, for an arbitrary azimuthal angle φ shown in the plan view of FIG. 2( a) in an actual three-dimensional model, and therefore the transmittance curves shown in FIG. 16 will drop for the entire range of the angle of incidence θ, rather than for the local range such as the arrows a, b, c, d and e, thus increasing the light-trapping effect of the light-coupling structures.

FIG. 17 shows results of an analysis using the sheet of the third embodiment, each showing the relationship between the angle of incidence θ of light on the end face of a single light-coupling structure and the transmittance thereof out of the light-trapping sheet. In the analysis conditions used, only the position of the light source S is shifted by 5 μm in the x-axis negative direction from the conditions of FIG. 15 or FIG. 3. FIG. 17( a) shows a case where the wavelength of the light source is λ=0.45 μm, FIG. 17( b) a case where the wavelength is λ=0.55 μm, and FIG. 17(c) a case where the wavelength is λ=0.65 μm, wherein each figure shows a comparison between the model of this embodiment and the model of Embodiment 1, and is also plotting the results obtained under a condition where there is no light-coupling structure (a configuration only with the light-transmitting sheet 2 and the light source S). A comparison between the results for the model of Embodiment 1 and the results (Nothing) obtained in a case where there is no light-coupling structure shows that they substantially coincide with each other in both cases within the critical angle (41.8° or less), but the latter is substantially zero and the former substantially floats outside the critical angle (41.8° or more). The former floats outside the critical angle because, as described above with reference to FIGS. 2( c) and 2(d), light incident on the end face of the first light-transmitting layer 3 a and the second light-transmitting layer 3 b of the light-coupling structure refracts, and then becomes narrow-angle light and is output from the upper surface. In contrast, with the results for the model of the third embodiment, the floating is significantly suppressed to be substantially zero in the range where the angle of incidence is 55° or more. This is because the first light-transmitting layer 3 a and the second light-transmitting layer 3 b account for no area on the end face of the third embodiment, and a component that is supposed to refract through the end face is totally reflected at the sloped surface 3 q of the second light-transmitting layer 3 b. Therefore, the third embodiment is a configuration such that the influence at the end face (the phenomenon that wide-angle light is converted to narrow-angle light) can be ignored more than in the first embodiment or the second embodiment, and can be said to be a configuration having a greater light-trapping effect.

The light-trapping sheet 53 can be manufactured by the following method, for example. FIGS. 18( a) to 18(f) are schematic cross-sectional configuration views showing a manufacturing procedure of the light-trapping sheet 53, and FIGS. 8( a) and 8(b) are schematic plan views each showing a pattern of a mold surface for producing the sheet. In FIG. 19( a), the surface of the mold 25 a is a plane, and rectangular minute structures 25A of the same size are two-dimensionally arranged, for example, on the surface of the mold 25 a. The rectangular minute structure 25A is a diffraction grating having a height of d and a pitch of Λ. The azimuth of the diffraction grating varies from one minute structure 25A to another. While diffraction gratings of 45°-interval azimuths, i.e., 0°, 45°, 90° and 135°, are arranged regularly in FIG. 19( a), it is preferred in practice that gratings are arranged with an equal frequency at smaller azimuths intervals, e.g., 30° or 15°. The rectangular minute structures 25B and 25B′ are two-dimensionally arranged also on the surfaces of the molds 25 b and 25 b′ of FIG. 19( b). The pitch of the arrangement of the minute structures 25B and 25B′ is equal to the pitch of the arrangement of the minute structures 25A. The minute structures 25B and 25B′ are depressed portions with planar bottoms. The depth of the depressed portion is equivalent to the dimension a or b of FIG. 14. The minute structures 25A of the mold 25 a are so large that their square shapes are almost in contact with one another (they may be in contact with one another), the minute structures 25B and 25B′ of the molds 25 b and 25 b′ are smaller.

As shown in FIG. 18( a), the transparent resin sheet 24 is laid on a mold 25 c having a flat surface and, with a thin layer of a spacer agent applied thereon, is pressed by the mold 25 a. As shown in FIG. 18( b), the mold 25 a is lifted to peel the mold 25 a off the resin sheet, and the flat resin sheet 24 a is laid on the resin sheet 24, onto which a diffraction grating has been transferred.

As shown in FIG. 18( c), the resin sheet 24 and the resin sheet 24 a are pressed by the mold 25 b while being heated, and the resin sheet 24 a is raised in the area of a depression 25B of the mold 25 b while attaching the resin sheet 24 and the resin sheet 24 a together in the other area. In this process, the diffraction grating is all buried to disappear in the attached portion, and remains only in the area where the resin sheet 24 a is raised. Raising the resin sheet 24 a forms an air layer (or a vacuum layer) between the resin sheet 24 a and the resin sheet 24. As shown in FIG. 18( d), the mold 25 c is lifted to peel the mold 25 c off the resin sheet 24, and a resin sheet 24 a′ is laid under the resin sheet 24. As shown in FIG. 18( e), the resin sheet 24 and the resin sheet 24 a′ are pressed by a mold 25 b′ while being heated, and the resin sheet 24 a′ is raised in the area of a depression 25B′ of the mold 25 b′ while attaching the resin sheet 24 and the resin sheet 24 a′ together in the other area. The rise of the resin sheet 24 a′ forms an air layer (or a vacuum layer) between the resin sheet 24 a′ and the resin sheet 24. As shown in FIG. 18( f), the molds 25 b and 25 b′ are peeled off, completing an attached sheet of the resin sheet 24 a, the resin sheet 24 and the resin sheet 24 a′. Thereafter, these attached sheets are bonded together via an adhesive layer therebetween, and the process is repeated, thereby producing the third region 2 c of the light-transmitting sheet 2 shown in FIG. 1( a). A resin sheet to be the first region 2 a and the second region 2 b of the light-transmitting sheet 2 is bonded to the front surface and the reverse surface of the third region 2 c of the light-transmitting sheet 2, thereby completing the light-trapping sheet 53. Note that anti-reflective nanostructures may be formed in advance on the surface of the resin sheet to be the resin sheets 24 a and 24 a″, the first region 2 a and the second region 2 b.

Fourth Embodiment

An embodiment of a light-receiving device according to the present invention will be described. FIG. 20 schematically shows a cross-sectional structure of a light-receiving device 54 of the present embodiment. The light-receiving device 54 includes the light-trapping sheet 51 of the first embodiment and a photoelectric conversion section 7. The light-trapping sheet 52 of the second embodiment or the light-trapping sheet 53 of the third embodiment may be used instead of the light-trapping sheet 51.

A reflective film 11 is preferably provided on end faces 2 s and 2 r of the light-trapping sheet 51. The photoelectric conversion section 7 is provided adjacent to the second principal surface 2 q of the light-trapping sheet 51. If the light-transmitting sheet 2 has a plurality of end faces, the reflective film 11 is preferably provided on all of the end faces. In the present embodiment, a portion of the second principal surface 2 q and a light-receiving portion of the photoelectric conversion section 7 are in contact with each other. The photoelectric conversion section 7 may be provided in a portion of the first principal surface 2 p of the light-trapping sheet 51.

By covering the end faces 2 r and 2 s of the light-trapping sheet 51 with the reflective film 11, light that has been taken and enclosed in the light-trapping sheet 51 will circulate in the light-trapping sheet 51.

The photoelectric conversion section 7 is a solar cell formed by a silicon. A plurality of photoelectric conversion sections 7 may be attached to one sheet of light-trapping sheet 51. Since the refractive index of silicon is about 5, even if light is made incident perpendicularly on the light-receiving surface of a solar cell, around 40% of the incident light is normally lost through reflection without being taken in the photoelectric conversion section 7. The reflection loss further increases when the light is incident diagonally. Although an AR coat or anti-reflective nanostructures are formed on the surface of a commercially-available solar cell in order to reduce the amount reflection, a sufficient level of performance has not been achieved. Moreover, a metal layer is present inside the solar cell, and a large portion of light that is reflected by the metal layer is radiated to the outside. With an AR coat or anti-reflective nanostructures, the reflected light is radiated to the outside with a high efficiency.

In contrast, the light-trapping sheet of the present invention takes in and encloses light for every visible light wavelength and for every angle of incidence in the light-trapping sheet. Therefore, with the light-receiving device 54, light entering through the first principal surface 2 p of the light-trapping sheet 51 is taken into the light-trapping sheet 51 and circulates in the light-trapping sheet 51. Since the refractive index of silicon is larger than the refractive index of the light-transmitting sheet 2, the wide-angle light 5 b′ and 6 b′ incident on the second principal surface 2 q are not totally reflected but portions thereof are transmitted into the photoelectric conversion section 7 as refracted light 5 d′ and 6 d′ and are converted to electric current in the photoelectric conversion section. After the reflected wide-angle light 5 c′ and 6 c′ propagate inside the photoelectric conversion section 7, they enter again and are used in photoelectric conversion until all the enclosed light is gone. Assuming that the refractive index of the transmissive sheet 2 is 1.5, the reflectance of light that is incident perpendicularly on the first principal surface 2 p is about 4%, but the reflectance can be suppressed to 1 to 2% or less, taking into account the wavelength dependency and the angle dependency, if an AR coat or anti-reflective nanostructures are formed on the surface thereof. Light other than this enters to be confined within the light-trapping sheet 51, and is used in photoelectric conversion.

With the light-receiving device of the present embodiment, most of the incident light can be confined within the sheet, most of which can be used in photoelectric conversion. Therefore, it is possible to significantly improve the energy conversion efficiency of the photoelectric conversion section. The light-receiving area is determined by the area of a first principal surface p, and all of the light received by this surface enters the photoelectric conversion section 7. Therefore, it is possible to reduce the area of the photoelectric conversion section 7 or reduce the number of photoelectric conversion sections 7, thereby realizing a significant cost reduction of the light-receiving device.

Fifth Embodiment

Another embodiment of a light-receiving device of the present invention will be described. FIG. 21 schematically shows a cross-sectional structure of a light-receiving device 55 of the present embodiment. The light-receiving device 55 includes the light-trapping sheet 51 of the first embodiment and the photoelectric conversion section 7. The light-trapping sheet 52 of the second embodiment or the light-trapping sheet 53 of the third embodiment may be used instead of the light-trapping sheet 51.

The light-receiving device 55 is different from the light-receiving device 54 of the fourth embodiment in that a protrusion/depression structure 8 is provided on the second principal surface 2 q, with a gap between the protrusion/depression structure 8 and the photoelectric conversion section 7. The protrusion/depression structure 8 provided on the second principal surface 2 q includes depressed portions and protruding portions whose width is 0.1 μm or more and which may be in a periodic pattern or a random pattern. With the protrusion/depression structure 8, the wide-angle light 5 b′ and 6 b′ incident on the second principal surface 2 q are not totally reflected, and portions thereof travel toward the photoelectric conversion section 7 as output light 5 d′ and 6 d′ to undergo photoelectric conversion. Light that are reflected by the surface of the photoelectric conversion section 7 are taken inside through the second principal surface 2 q of the light-trapping sheet 51 and propagates inside the light-trapping sheet 51, after which the light again travel toward the photoelectric conversion section 7 as the output light 5 d′ and 6 d′. Therefore, also with the light-receiving device of the present embodiment, most of the incident light can be confined within the light-trapping sheet, most of which can be used in photoelectric conversion. As in the fourth embodiment, it is possible to reduce the area of the photoelectric conversion section 7 or reduce the number of photoelectric conversion sections 7. Therefore, it is possible to realize a light-receiving device having a significantly improved energy conversion efficiency and being capable of cost reduction.

Sixth Embodiment

Another embodiment of a light-receiving device of the present invention will be described. FIG. 22 schematically shows a cross-sectional structure of a light-receiving device 56 of the present embodiment. The light-receiving device 56 includes the light-trapping sheet 51 of the first embodiment, the photoelectric conversion section 7, and a prism sheet 9. The light-trapping sheet 52 of the second embodiment or the light-trapping sheet 53 of the third embodiment may be used instead of the light-trapping sheet 51.

The light-receiving device 56 is different from the light-receiving device 54 of the fourth embodiment in that the prism sheet 9 is provided between the second principal surface 2 q and the photoelectric conversion section 7. Tetrahedron prisms 10 are arranged adjacent to one another inside the prism sheet 9. The prism sheet 9 may be formed by layering together two triangular prism array sheets orthogonal to each other. Since the refractive index of the prism 10 is set to be larger than the refractive index of the prism sheet 9, the wide-angle light 5 b′ and 6 b′ incident on the surface of the prism sheet 9 are refracted by the prism surface to be 5 d′ and 6 d′ and travel toward the photoelectric conversion section 7. Since the angle of incidence of light to the photoelectric conversion section 7 is close to perpendicular, it is possible to reduce the reflection at the light-receiving surface of the photoelectric conversion section 7 and to reduce the number of light circulations within the light-trapping sheet 51 as compared with the fourth embodiment.

Also with the light-receiving device of the present embodiment, most of the incident light can be confined within the light-trapping sheet, most of which can be used in photoelectric conversion. As in the fourth embodiment, it is possible to reduce the area of the photoelectric conversion section 7 or reduce the number of photoelectric conversion sections 7. Therefore, it is possible to realize a light-receiving device having a significantly improved energy conversion efficiency and being capable of cost reduction. Since the number of light circulations within the sheet is smaller than the fourth embodiment, it is less influenced by the light-trapping capacity of the light-trapping sheet.

Seventh Embodiment

Another embodiment of a light-receiving device of the present invention will be described. FIG. 23 schematically shows a cross-sectional structure of a light-receiving device 57 of the present embodiment. The light-receiving device 57 includes the light-trapping sheet 51 of the first embodiment and the photoelectric conversion section 7. The light-trapping sheet 52 of the second embodiment or the light-trapping sheet 53 of the third embodiment may be used instead of the light-trapping sheet 51.

The light-receiving device 57 is different from the light-receiving device 54 of the fourth embodiment in that the end faces 2 s and 2 r are covered by the photoelectric conversion section 7 instead of the reflective film 11. If the light-transmitting sheet 2 has a plurality of end faces, the photoelectric conversion section 7 is preferably provided on all of the end faces. In the present embodiment, the fourth region 2 h may be absent in the light-trapping sheet 51.

When the photoelectric conversion section 7 is provided on the end faces 2 s and 2 r, the wide-angle light 5 c, 6 c, 5 c′ and 6 c′ enter the photoelectric conversion section 7 along the normal to the light-receiving surface of the photoelectric conversion section 7, as opposed to the fourth embodiment. Therefore, there is less reflection at the surface of the photoelectric conversion section 7, and it is possible to reduce the number of light circulations within the light-trapping sheet 51.

Also with the light-receiving device of the present embodiment, most of the incident light can be confined within the light-trapping sheet, most of which can be used in photoelectric conversion. Therefore, it is possible to realize a light-receiving device having a significantly improved energy conversion efficiency. Since the area of the photoelectric conversion section 7 can be reduced as compared with the fourth embodiment, it is possible to significantly reduce the cost. Since the number of light circulations within the sheet is smaller than the fourth embodiment, it is less influenced by the light-trapping capacity of the light-trapping sheet.

Eighth Embodiment

Another embodiment of a light-receiving device of the present invention will be described. FIG. 24 schematically shows a cross-sectional structure of a light-receiving device 58 of the present embodiment. The light-receiving device 58 includes light-trapping sheets 51 and 51′, and the photoelectric conversion section 7. The first light-trapping sheet 51, the light-trapping sheet 52 of the second embodiment or the light-trapping sheet 53 of the third embodiment may be used, independently, instead of the light-trapping sheets 51 and 51′. In the present embodiment, the fourth region 2 h may be absent in the light-trapping sheet 51′.

The light-receiving device 58 is different from the fourth embodiment in that the attachment is such that the end face 2 s of the light-trapping sheet 51 is in contact with the first principal surface 2 p of the light-receiving device 54 of the fourth embodiment. The light-trapping sheet 51′ is preferably attached orthogonal to the light-trapping sheet 51. In the light-trapping sheet 51′, it is preferred that the reflective film 11 is provided on the end face 2 r, and a reflective film 11′ is provided on a first principal surface 2 p′ and a second principal surface 2 q′ in the vicinity of the end face 2 s which is attached to the light-trapping sheet 51. The reflective film 11′ serves to reflect the light 6 b so as to prevent the wide-angle light 6 b from the light-trapping sheet 51 from leaking out of the light-trapping sheet 51′.

The light 4 incident on the first principal surface 2 p of the light-trapping sheet 51 is taken into the light-trapping sheet 51. On the other hand, light 4′ incident on the first principal surface 2 p′ and the second principal surface 2 q′ of the light-trapping sheet 51′ is taken into the light-trapping sheet 51′. Light taken into the light-trapping sheet 51′ becomes guided light 12 propagating toward the end face 2 s, since the end face 2 r is covered by the reflective film 11, and merges with the light inside the light-trapping sheet 51. Since a portion of the second principal surface 2 q in the light-trapping sheet 51 is in contact with the surface of the photoelectric conversion section 7, and the refractive index of silicon is larger than the refractive index of the light-transmitting sheet 2, the wide-angle light 5 b′ and 6 b′ incident on the second principal surface 2 q are not totally reflected but portions thereof are incident on the photoelectric conversion section 7 as the refracted light 5 d′ and 6 d′ and are converted to electric current in the photoelectric conversion section 7. The reflected wide-angle light 5 c′ and 6 c′ propagate inside the light-trapping sheet 51, are incident again on the light-receiving surface of the photoelectric conversion section 7, and are used in photoelectric conversion until the enclosed light is mostly gone.

Since the light-receiving device of the present embodiment includes the light-trapping sheet 51′ perpendicular to the light-receiving surface of the photoelectric conversion section 7, even light that is incident diagonally on the first principal surface 2 p of the light-trapping sheet 51 is incident, at an angle close to perpendicular, on the first principal surface 2 p′ and the second principal surface 2 q′ of the light-trapping sheet 51′. This makes it easier to take in light of every azimuth.

Also with the light-receiving device of the present embodiment, most of the incident light can be confined within the light-trapping sheet, most of which can be used in photoelectric conversion. As in the fourth embodiment, it is possible to reduce the area of the photoelectric conversion section 7 or reduce the number of photoelectric conversion sections 7. Therefore, it is possible to realize a light-receiving device having a significantly improved energy conversion efficiency and being capable of cost reduction.

Ninth Embodiment

An embodiment of a lighting plate according to the present invention will be described. FIG. 25 schematically shows a cross-sectional structure of a lighting plate 59 of the present embodiment. The lighting plate 59 includes the light-trapping sheet 51 of the first embodiment, and the protrusion/depression structure 8 provided on portions of the first principal surface 2 p and the second principal surface 2 q of the light-trapping sheet 51. The light-trapping sheet 52 of the second embodiment or the light-trapping sheet 53 of the third embodiment may be used instead of the light-trapping sheet 51. In the light-trapping sheet 51, the reflective film 11 is provided on the end faces 2 r and 2 s.

The protrusion/depression structure 8 is formed on a portion of the first principal surface 2 p, forms a random pattern of depressed portions and protruding portions whose width is 0.1 μm or more. Light taken into the light-trapping sheet 51 propagates inside the light-trapping sheet 51, and portions of the propagating light are radiated outside as the output light 5 d′ and 6 d′ by the protrusion/depression structure 8.

The lighting plate 59 is provided on a window for lighting of a building such as a house so that the first principal surface 2 p with the protrusion/depression structure 8 provided thereon is facing the room side. During the day, the lighting plate 59 takes in the light of the sun 13 a through the second principal surface 2 q, and radiates it into the room through the protrusion/depression structure 8. Thus, it can be used as an indoor lighting in which light is radiated from the protrusion/depression structure 8. During the night, the lighting plate 59 takes in light from an indoor lighting 13 b through the first principal surface 2 p, and radiates the light through the protrusion/depression structure 8. Thus, the lighting plate 59 can be used as an auxiliary to an indoor lighting. Thus, with the lighting plate of the present embodiment, it is possible to confine most of the incident light within the sheet, and reuse it as a lighting, thereby realizing an efficient use of energy.

Tenth Embodiment

An embodiment of a light-emitting device according to the present invention will be described. FIG. 26 schematically shows a cross-sectional structure of a light-emitting device 60 of the present embodiment. The light-emitting device 60 includes the light-trapping sheet 51, a light source 14, and the prism sheet 9. The light-trapping sheet 52 of the second embodiment or the light-trapping sheet 53 of the third embodiment may be used instead of the light-trapping sheet 51.

The light source 14, such as an LED, is provided adjacent to one of the first principal surface 2 p and the second principal surface 2 q of the light-trapping sheet 51, with the protrusion/depression structure 8 provided on the other. In the present embodiment, the light source 14 is provided adjacent to the first principal surface 2 p, and the protrusion/depression structure 8 is provided on the second principal surface 2 q. The reflective film 11 is provided on the end faces 2 s and 2 r of the light-trapping sheet 51. The protrusion/depression structure 8 includes depressed portions and protruding portions whose width is 0.1 μm or more and which may be in a periodic pattern or a random pattern.

The prism sheet 9 is arranged with a gap from the second principal surface 2 q so as to oppose the protrusion/depression structure 8. The tetrahedron prisms 10 are arranged adjacent to one another inside the prism sheet 9. The prism sheet 9 may be formed by layering together two triangular prism array sheets orthogonal to each other.

The light 4 output from the light source 14 is taken in through the first principal surface 2 p of the light-trapping sheet 51 to be the light 12 that propagates inside the light-trapping sheet 51. Portions of this light are radiated outside as the output light 5 d′ and 6 d′ by the protrusion/depression structure 8. The radiated light is condensed through the prisms 10 inside the prism sheet 9 to be light 4 a having a substantially parallel wave front.

With the light-emitting device of the present embodiment, it is possible, with a simple and thin configuration, to confine light output from a point light source into a light-trapping sheet, and take out the light as a surface light source.

Eleventh Embodiment

An embodiment of a light-trapping rod according to the present invention will be described. FIGS. 27( a) and 27(b) schematically show a cross-sectional structure of a light-trapping rod 61 of the present embodiment parallel to the central axis, and a cross-sectional structure thereof perpendicular to the central axis. The light-trapping rod 61 includes a light-transmitting rod 2′, and at least one light-coupling structure 3 arranged inside the light-transmitting rod 2′.

The light-transmitting rod 2′ has a circular or elliptical cross-sectional shape on a plane that is perpendicular to the central axis C. The light-transmitting rod 2′ is formed by a transparent material that transmits therethrough light of a desired wavelength or light within a desired wavelength range determined according to the application, as in the first embodiment.

Where the cross section of the light-transmitting rod 2′ is circular, the diameter D of the light-transmitting rod 2′ on a cross section perpendicular to the central axis C is about 0.05 mm to 2 mm, for example. One or more light-coupling structures 3 are provided at a distance of d3 or more from a surface 2 u, which is the principal surface of the light-transmitting rod 2′, in the direction toward the central axis C. Preferably, the light-trapping rod 61 includes a plurality of coupling structures 3. The light-transmitting rod 2′ has a circular or elliptical cross-sectional shape, and the light-coupling structures 3 are arranged within a core region 2A that has a circular shape having a diameter of d=D−2×d3 centered about the central axis C on a plane that is perpendicular to the central axis C of the light-transmitting rod 2′ and that is extending along the central axis C direction.

The light-coupling structures 3 are arranged within the core region 2A at a predetermined density in the axial direction, the radial direction and the circumferential direction. The density at which the light-coupling structures 3 are arranged is, for example, 10 to 10³ per 1 mm in the axial direction, 10 to 10³ per 1 mm in the radial direction, and 10 to 10³ per 1 mm in the circumferential direction. The cross-sectional shape of the core region is circular or elliptical, and may be a shape with two or more rings.

The light-coupling structures 3 have the same structure as that of the light-coupling structures 3 of the first embodiment. The light-trapping rod 61 may include the light-coupling structures 3′ of the second embodiment or the light-coupling structures 3″ of the third embodiment, instead of the light-coupling structures 3.

The light-coupling structures 3 is arranged within the core region 2A so that the diffraction grating of the third light-transmitting layer 3 c is parallel to the central axis C of the light-transmitting rod 2′. The length L of the light-coupling structure 3 in the central axis C direction is 3 μm to 100 μm, and the length W thereof in the direction orthogonal thereto is about ⅓ to 1/10 of L.

In FIGS. 27( a) and 27(b), it is assumed that the refractive index of the environmental medium surrounding the light-trapping rod 61 is 1.0, and the refractive index of the light-transmitting rod 2′ is n_(s). The light 4 from the environmental medium passes through the surface 2 u and enters the inside of the light-transmitting rod 2′. An AR coat or anti-reflective nanostructures (e.g., moth-eye structures) may be formed on the surface 2 u in order to increase the transmittance of the incident light 4. Herein, of the light inside the light-transmitting rod 2′, light that satisfies sin θ<1/n_(s) will be referred to as the narrow-angle light and light that satisfies sin θ1/n_(s) as the wide-angle light, regarding the angle θ (propagation angle) formed between the propagation azimuth and the normal to the rod surface.

First, light vectors on a cross section parallel to the central axis C of the light-transmitting rod 2′ will be discussed. In this cross section, a portion of the narrow-angle light 5 a inside the light-transmitting rod 2 is converted by a light-coupling structure 3 to the wide-angle light 5 b, and this light is totally reflected by the surface 2 u to be the wide-angle light 5 c which stays inside the light-transmitting rod 2′. A portion of the remaining narrow-angle light 5 a′ of the narrow-angle light 5 a is converted by another light-coupling structure 3 to the wide-angle light 5 b′, and this light is totally reflected by the surface 2 u to be the wide-angle light 5 c′ which stays inside the rod. In this manner, all of the narrow-angle light 5 a is converted to the wide-angle light 5 b or 5 b′ within the core region 2A where the light-coupling structures 3 are provided. On the other hand, a portion of the wide-angle light 6 a inside the light-transmitting rod 2′ is totally reflected by the surface of a light-coupling structure 3 to be the wide-angle light 6 b, and this light is totally reflected by the surface 2 u to be the wide-angle light 6 c which stays within the rod. A portion of the remaining light of the wide-angle light 6 a passes through the core region 2A where the light-coupling structures 3 are provided, and this wide-angle light 6 b′ is totally reflected at the surface 2 u to be the wide-angle light 6 c′ which stays within the light-transmitting rod 2′. Although not shown in the figure, there is also wide-angle light that stays within the sheet while being totally reflected between different light-coupling structures 3 and between the surfaces 2 u.

As described above with reference to FIG. 2( a), the narrow-angle light 5 a passes through the surface 3 q of the second light-transmitting layer 3 b, and a portion thereof is converted to the guided light 5B which propagates inside the third light-transmitting layer 3 c by the function of the diffraction grating. The remainder becomes transmitted light or diffracted light, and it primarily becomes the narrow-angle light 5 a′ to pass through the light-coupling structures 3, or becomes the narrow-angle light 5 r as reflected light to pass through the light-coupling structures 3. Before the guided light 5B reaches the end face 3S of the third light-transmitting layer 3 c, a portion thereof is radiated in the same direction as 5 r within the critical angle to be the narrow-angle light 5 r′, and the remainder is guided and radiated from the end face 3S of the third light-transmitting layer 3 c to be the wide-angle light 5 c. On the other hand, the wide-angle light 6 a is totally reflected at the surface 3 q of the second light-transmitting layer 3 b, all of which becomes the wide-angle light 6 b. Thus, wide-angle light that is incident on the surface of the light-coupling structure 3 (the surface 3 p of the first light-transmitting layer 3 a and the surface 3 q of the second light-transmitting layer 3 b) remains to be wide-angle light, whereas a portion of narrow-angle light incident thereon is converted to the wide-angle light.

Next, light vectors on a cross section orthogonal to the central axis of the rod will be discussed. On this cross section, light entering inside the rod are classified into three types. These are light 15 a passing through the core region 2A, light 15 b passing through the outer edge of the core region 2A, and light 15 c passing through the outside of the core region 2A. The light 15 a is converted to wide-angle light which stays within the rod on the cross section along the central axis of the rod as described above. On the other hand, the light 15 b is light that is incident at an angle of ψ on the surface 2 u of the rod, where ψ satisfies Expression 3.

$\begin{matrix} {\left\lbrack {{Exp}.\mspace{14mu} 3} \right\rbrack\mspace{661mu}} & \; \\ {{\sin\;\psi} = \frac{d}{D}} & (3) \end{matrix}$

Naturally, the angle of incidence of the light 15 c on the surface 2 u is greater than ψ. Therefore, if Expression 4 holds true, the light 15 b is totally reflected by the first principal surface 2 p of the rod, and the light 15 b and 15 c become wide-angle light which stays within the light-transmitting rod 2′ on the cross section orthogonal to the central axis.

$\begin{matrix} {\left\lbrack {{Exp}.\mspace{14mu} 4} \right\rbrack\mspace{661mu}} & \; \\ {\frac{d}{D} \geq \frac{1}{n_{0}}} & (4) \end{matrix}$

Therefore, satisfying Expression 4 for both the cross section parallel to the central axis C of the light-transmitting rod 2′ and the cross section orthogonal thereto is the condition for all the light inside the light-transmitting rod 2′ to stay within the light-transmitting rod 2′.

FIG. 28 is a schematic cross-sectional configuration view showing a production procedure for the light-trapping rod 61. In FIG. 28, the resin sheet 24, 24 a (and 24′, 24 a′) shown in FIG. 7, 13, 18 is produced by the same method as those of the first to third embodiments. As to the grating vectors of the diffraction gratings forming the light-coupling structures 3 on the resin sheet 24, 24 a (and 24 a′), diffraction gratings of various pitches may be combined together, or diffraction gratings of a single pitch may be arranged so as to be in various directions (e.g., angles at 30° or 15° intervals) with respect to the z-axis, or a combination thereof may be used, so that the pitch as measured along the z-axis is from 0.30 μm to 2.80 μm. As to the size of the light-coupling structures 3, the length L in the z-axis direction is set to be 3 μm to 100 μm and the length W in the direction orthogonal thereto is set to be about ½ to 1/10 of L so that the coupled guided light can be radiated as much as possible along the central axis of the rod. The core region 2A of the light-trapping rod 61 can be produced by rolling up this sheet about the z axis with thin layer of an adhesive applied on one surface thereof where the diffraction gratings are absent. The light-trapping rod 61 is completed by wrapping it with a transparent protection layer with anti-reflective nanostructures formed thereon.

Twelfth Embodiment

An embodiment of a light-emitting device according to the present invention will be described. FIG. 29 schematically shows a cross-sectional structure of a light-emitting device 62 of the present embodiment. The light-emitting device 62 includes the light-trapping rod 61, and light sources 14R, 14G and 14B. The light-trapping rod 61 has such a structure as described above in the eleventh embodiment.

The reflective film 11 is provided on the end face 2 r of the light-trapping rod 61. Preferably, a tapered portion 2 v is provided on the surface 2 u of the light-trapping rod 61 on the side of the end face 2 s, and a waveguide 18 having a smaller diameter than the light-transmitting rod 2′ is connected thereto.

The light sources 14R, 14G and 14B are formed by LDs and LEDs, and output red, green and blue light, respectively, for example. Light output from these light sources are condensed through lenses to radiate light 4R, 4G and 4B toward the surface 2 u of the light-transmitting rod 2′. These light are confined inside the light-transmitting rod 2′ by the light-coupling structures 3 in the core region 2A, and since the end face 2 r is covered by the reflective film 11, it as a whole becomes the guided light 12 which propagates in one direction inside the rod. The guided light 12 is narrowed with no loss through the tapered portion 2 v over which the diameter of the rod 2′ decreases gradually, and it becomes guided light which propagates inside the waveguide 18 having a narrow diameter. Thus, the light 19, which is close to a point light source, is output from the end face of the waveguide 18. Where the light sources are lasers, the light 4R, 4G and 4B are coherent light, but since the light are radiated from the individual light-coupling structures 3 in varied phases, the guided light 12 obtained by synthesizing the radiated light together will incoherent light. Therefore, the output light 19 is also incoherent light. By adjusting the light amounts of the light 4R, 4G and 4B, the output light 19 can be made white light. At present, red and blue semiconductor lasers have been realized, and a green laser is also available by using SHG. Synthesizing white light from these light sources typically requires a complicated optical configuration, and results in glaring light due to the coherence characteristic of laser light. However, with the light-emitting device 62 of the present embodiment, it is possible to provide a more natural, white-light point light source with no glare with a very simple configuration.

In the case of the present embodiment, what needs an adjustment is the positional adjustment between the convergent light formed by the incident light 4R, 4G and 4B and the rod 2′. FIG. 30 is a cross-sectional view showing how light is incident on the light-trapping rod 61, where point O is the center of the rod. Assuming that the refractive index of the light-transmitting rod 2′ is 1.5, the light 16 a parallel to the straight line AOB refracts to be light 16 b that is condensed approximately at point A. Assuming that the diameter of the core region 2A is greater than 1/1.5 the diameter of the light-transmitting rod 2′, based on Expression 4, the light 16 b certainly passes through the core region 2A to be confined within the light-transmitting rod 2′. In contrast, it is difficult to draw a light ray that does not pass through the core region 2A. For example, as for light 17 b that is incident on point B and does not pass through the core region, the incident light beam 17 a thereof is light at a very small angle with respect to the surface of incidence (light at an outermost edge of a condensation realized by a high numerical aperture). In other words, light rays at typical angles of incidence, i.e., light obtained by a condensation realized by a typical numerical aperture, all pass through the core region 2A to be confined within the light-transmitting rod 2′. This indicates that the positional adjustment between the incident light 4R, 4G and 4B and the light-transmitting rod 2′ can be very rough, thus indicating how easy the adjustment is.

Thirteenth Embodiment

Another embodiment of a light-emitting device according to the present invention will be described. FIG. 31 schematically shows a cross-sectional structure of a light-emitting device 63 of the present embodiment. The light-emitting device 63 includes the light-trapping rod 61, the light source 14, and the prism sheet 9. The light-trapping rod 61 has such a structure as described above in the eleventh embodiment.

The reflective film 11 is provided on the end face 2 r of the light-trapping rod 61. A portion of the light-trapping rod 61 where the light-coupling structures 3 are absent functions as the waveguide 18. The prism sheet 9 is provided on the surface 2 u of the waveguide 18.

The light source 14 is formed by an LD, an LED, or the like, and emits visible light. The light output from the light source is condensed through a lens to be the light 4 passing through the light-transmitting rod 2′. These light are confined inside the light-transmitting rod 2′ by the light-coupling structures 3 in the core region 2A, and since one of the end faces is covered by the reflective film 11, it as a whole becomes the light 12 which propagates in one direction inside the light-transmitting rod 2′, and becomes guided light which propagates inside the waveguide 18. The prism sheet 9 is provided in contact with the waveguide 18. The tetrahedron prisms 10 are arranged adjacent to one another inside the prism sheet 9. It may be formed by triangular prism array sheets orthogonal to each other that are bonded together. Since the refractive index of the prism 10 is larger than the refractive index of the prism sheet 9, light leaking out of the waveguide 18 to be incident on the prism sheet 9 refracts and is output from the prism sheet 9 to be the parallel output light 19. Note that the prism sheet 9 may be separated from the waveguide 18, in which case a protrusion/depression structure is formed on one side of the surface of the waveguide 18 that is opposing the prism sheet 9 for outputting light therethrough.

Where the light source is a laser, the light 4 is coherent light, but since the light are radiated from the individual light-coupling structures 3 in varied phases, the guided light 12 obtained by synthesizing the radiated light together will be incoherent light. Therefore, the output light 19 is also incoherent light. At present, red and blue semiconductor lasers have been realized, and a green laser is also available by using SHG. Using these light sources, red, green and blue linear light sources are obtained. For example, by bundling together these linear light sources, it is possible to provide a color backlight for a liquid crystal display with a very simple configuration.

INDUSTRIAL APPLICABILITY

Sheets and rods of the present invention are capable of taking in light over a wide area, and over a wide wavelength range (e.g., the entire visible light range) for every angle of incidence; therefore, light-receiving devices using the same are useful as high-conversion-efficiency solar cells, or the like, and light-receiving and light-emitting devices using the same provide a new form of a lighting or a light source, and are useful as a recycle lighting using the sunlight or light from a lighting, a high-efficiency backlight, and an incoherent white light source.

REFERENCE SIGNS LIST

-   -   2 Light-transmitting sheet     -   2′ Light-transmitting rod     -   2 p First principal surface     -   2 q Second principal surface     -   2 u Surface     -   3, 3′, 3″Light-coupling structure     -   3 a First light-transmitting layer     -   3 b Second light-transmitting layer     -   3 c Third light-transmitting layer     -   3 d Diffraction grating     -   4 Incident light     -   5 a, 5 a′ Narrow-angle light     -   5 b, 5 c, 5 b′, 5 c′ Wide-angle light     -   6 a, 6 b, 6 c, 6 b′, 6 c′ Wide-angle light     -   9 Prism sheet     -   10 Prism     -   11 Reflective film     -   14 Light source 

The invention claimed is:
 1. A light-trapping sheet comprising: a light-transmitting sheet having first and second principal surfaces; and a plurality of light-coupling structures arranged in an inner portion of the light-transmitting sheet, the inner portion being located at a first distance or more and a second distance or more from the first and second principal surfaces, respectively, wherein: each of the plurality of light-coupling structures includes a first light-transmitting layer, a second light-transmitting layer, and a third light-transmitting layer arranged therebetween; refractive indecies of the first and second light-transmitting layers are smaller than a refractive index of the light-transmitting sheet; a refractive index of the third light-transmitting layer is larger than the refractive indecies of the first and second light-transmitting layers; and the third light-transmitting layer has a diffraction grating parallel to the first and second principal surfaces of the light-transmitting sheet; the plurality of light-coupling structures include a first light-coupling structure and a second light-coupling structure arranged next to each other on a surface parallel to the first and second principal surfaces, and the first light-transmitting layer of the first light-coupling structure is spaced apart from the first light-transmitting layer of the second light-coupling structure and the second light-transmitting layer of the first light-coupling structure is spaced apart from the second light-transmitting layer of the second light-coupling structure.
 2. The light-trapping sheet of claim 1, wherein the plurality of light-coupling structures are arranged three-dimensionally in an inner portion of the light-transmitting sheet at the first distance or more and the second distance or more from the first and second principal surfaces, respectively.
 3. The light-trapping sheet of claim 2, wherein: a pitch of the diffraction grating is 0.1 μm or more and 3 μm or less; surfaces of the first and second light-transmitting layers are each sized so as to circumscribe a circle having a diameter of 100 μm or less; and the plurality of light-coupling structures each have a thickness of 3 μm or less.
 4. The light-trapping sheet of claim 3, wherein at least two of the plurality of light-coupling structures are different from each other in terms of a direction in which the diffraction grating extends.
 5. The light-trapping sheet of claim 3, wherein at least two of the plurality of light-coupling structures are different from each other in terms of a pitch of the diffraction grating.
 6. The light-trapping sheet of claim 1, wherein: the light-transmitting sheet includes: a first region being in contact with the first principal surface and having a thickness equal to the first distance; a second region being in contact with the second principal surface and having a thickness equal to the second distance; a third region arranged between the first and second regions; and at least one fourth region provided in the third region and connecting the first region and the second region to each other; the plurality of light-coupling structures are arranged only in the third region excluding the at least one fourth region; and an arbitrary straight line passing through the fourth region is extending along an angle greater than a critical angle, which is defined by the refractive index of the light-transmitting sheet and a refractive index of an environmental medium surrounding the light-transmitting sheet, with respect to a thickness direction of the light-transmitting sheet.
 7. The light-trapping sheet of claim 1, wherein in at least one of the plurality of light-coupling structures, thicknesses of the first and second light-transmitting layers are decreased toward an outer edge side away from a center of the light-coupling structure.
 8. The light-trapping sheet of claim 1, wherein in at least one of the plurality of light-coupling structures, a protrusion/depression structure whose pitch and height are ⅓ or less of a design wavelength is formed on one of surfaces of the first and second light-transmitting layers that are in contact with the light-transmitting sheet, the first principal surface, and the second principal surface.
 9. A light-trapping rod comprising: a light-transmitting rod having a principal surface and a circular or elliptical cross section; and a plurality of light-coupling structures arranged in an inner portion of the light-transmitting rod, the inner portion being located at a first distance or more from the principal surface, wherein: each of the plurality of light-coupling structures includes a first light-transmitting layer, a second light-transmitting layer, and a third light-transmitting layer arranged therebetween; refractive indecies of the first and second light-transmitting layers are smaller than a refractive index of the light-transmitting rod; a refractive index of the third light-transmitting layer is larger than the refractive indecies of the first and second light-transmitting layers; and the third light-transmitting layer includes a diffraction grating parallel to a central axis of the light-transmitting rod; the plurality of light-coupling structures include a first light-coupling structure and a second light-coupling structure arranged next to each other on a surface parallel to the first and second principal surfaces, and the first light-transmitting layer of the first light-coupling structure is spaced apart from the first light-transmitting layer of the second light-coupling structure and the second light-transmitting layer of the first light-coupling structure is spaced apart from the second light-transmitting layer of the second light-coupling structure.
 10. The light-trapping rod of claim 9, wherein the plurality of light-coupling structures are each arranged three-dimensionally in an inner portion of the light-transmitting rod at the first distance or more from the principal surface.
 11. The light-trapping rod of claim 10, wherein: a pitch of the diffraction grating is 0.1 μm or more and 3 μm or less; surfaces of the first and second light-transmitting layers are each sized so as to circumscribe a circle having a diameter of 100 μm or less; and the light-coupling structures each have a thickness of 3 μm or less.
 12. The light-trapping rod of claim 11, wherein at least two of the plurality of light-coupling structures are different from each other in terms of a direction in which the diffraction grating extends.
 13. The light-trapping rod of claim 11, wherein at least two of the plurality of light-coupling structures are different from each other in terms of a pitch of the diffraction grating.
 14. The light-trapping rod of claim 9, wherein in at least one of the plurality of light-coupling structures, a protrusion/depression structure whose pitch and height are ⅓ or less of a design wavelength is formed on one of surfaces of the first and second light-transmitting layers that are in contact with the light-transmitting rod, and the principal surface.
 15. A light-receiving device comprising: a light-trapping sheet of claim 1; a photoelectric conversion section provided on one of the first principal surface of the light-trapping sheet, the second principal surface thereof, and end faces adjacent to the first principal surface and the second principal surface.
 16. The light-receiving device of claim 15, further comprising: another light-trapping sheet of claim 1, wherein: the photoelectric conversion section is provided on the first principal surface of the light-trapping sheet; and an end face of the other light-trapping sheet is connected to the second principal surface of the light-trapping sheet.
 17. A light-receiving device comprising: a light-trapping sheet of claim 1; and a protrusion/depression structure or a prism sheet provided on at least a portion of the first principal surface or the second principal surface of the light-trapping sheet.
 18. A light-emitting device comprising: a light-trapping sheet of claim 1; a light source provided adjacent to one of the first principal surface and the second principal surface of the light-trapping sheet; a protrusion/depression structure provided on the other one of the first principal surface and the second principal surface of the light-trapping sheet; and a prism sheet arranged so as to receive light output from the protrusion/depression structure.
 19. A light-emitting device comprising: a light-trapping rod of claim 9; and at least one light source provided adjacent to the first principal surface of the light-transmitting rod.
 20. The light-emitting device of claim 19, further comprising a prism sheet or a protrusion/depression structure provided on a portion of the first principal surface of the light-transmitting rod. 